diff --git a/lib/mv/membership_fees/calendar_cycles.ex b/lib/mv/membership_fees/calendar_cycles.ex
new file mode 100644
index 0000000..8a4ef24
--- /dev/null
+++ b/lib/mv/membership_fees/calendar_cycles.ex
@@ -0,0 +1,329 @@
+defmodule Mv.MembershipFees.CalendarCycles do
+  @moduledoc """
+  Calendar-based cycle calculation functions for membership fees.
+
+  This module provides functions for calculating cycle boundaries
+  based on interval types (monthly, quarterly, half-yearly, yearly).
+
+  The calculation functions (`calculate_cycle_start/3`, `calculate_cycle_end/2`,
+  `next_cycle_start/2`) are pure functions with no side effects.
+
+  The time-dependent functions (`current_cycle?/3`, `last_completed_cycle?/3`)
+  depend on a date parameter for testability. Their 2-argument variants
+  (`current_cycle?/2`, `last_completed_cycle?/2`) use `Date.utc_today()` and
+  are not referentially transparent.
+
+  ## Interval Types
+
+  - `:monthly` - Cycles from 1st to last day of each month
+  - `:quarterly` - Cycles from 1st of Jan/Apr/Jul/Oct to last day of quarter
+  - `:half_yearly` - Cycles from 1st of Jan/Jul to last day of half-year
+  - `:yearly` - Cycles from Jan 1st to Dec 31st
+
+  ## Examples
+
+      iex> date = ~D[2024-03-15]
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_start(date, :monthly)
+      ~D[2024-03-01]
+
+      iex> cycle_start = ~D[2024-01-01]
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_end(cycle_start, :yearly)
+      ~D[2024-12-31]
+
+      iex> cycle_start = ~D[2024-01-01]
+      iex> Mv.MembershipFees.CalendarCycles.next_cycle_start(cycle_start, :yearly)
+      ~D[2025-01-01]
+  """
+
+  @typedoc """
+  Interval type for membership fee cycles.
+
+  - `:monthly` - Monthly cycles (1st to last day of month)
+  - `:quarterly` - Quarterly cycles (1st of quarter to last day of quarter)
+  - `:half_yearly` - Half-yearly cycles (1st of half-year to last day of half-year)
+  - `:yearly` - Yearly cycles (Jan 1st to Dec 31st)
+  """
+  @type interval :: :monthly | :quarterly | :half_yearly | :yearly
+
+  @doc """
+  Calculates the start date of the cycle that contains the reference date.
+
+  ## Parameters
+
+    - `date` - Ignored in this 3-argument version (kept for API consistency)
+    - `interval` - The interval type (`:monthly`, `:quarterly`, `:half_yearly`, `:yearly`)
+    - `reference_date` - The date used to determine which cycle to calculate
+
+  ## Returns
+
+    The start date of the cycle containing the reference date.
+
+  ## Examples
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_start(~D[2024-03-15], :monthly, ~D[2024-05-20])
+      ~D[2024-05-01]
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_start(~D[2024-03-15], :quarterly, ~D[2024-05-20])
+      ~D[2024-04-01]
+  """
+  @spec calculate_cycle_start(Date.t(), interval(), Date.t()) :: Date.t()
+  def calculate_cycle_start(_date, interval, reference_date) do
+    case interval do
+      :monthly -> monthly_cycle_start(reference_date)
+      :quarterly -> quarterly_cycle_start(reference_date)
+      :half_yearly -> half_yearly_cycle_start(reference_date)
+      :yearly -> yearly_cycle_start(reference_date)
+    end
+  end
+
+  @doc """
+  Calculates the start date of the cycle that contains the given date.
+
+  This is a convenience function that calls `calculate_cycle_start/3` with `date` as both
+  the input and reference date.
+
+  ## Parameters
+
+    - `date` - The date used to determine which cycle to calculate
+    - `interval` - The interval type (`:monthly`, `:quarterly`, `:half_yearly`, `:yearly`)
+
+  ## Returns
+
+    The start date of the cycle containing the given date.
+
+  ## Examples
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_start(~D[2024-03-15], :monthly)
+      ~D[2024-03-01]
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_start(~D[2024-05-15], :quarterly)
+      ~D[2024-04-01]
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_start(~D[2024-09-15], :half_yearly)
+      ~D[2024-07-01]
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_start(~D[2024-12-15], :yearly)
+      ~D[2024-01-01]
+  """
+  @spec calculate_cycle_start(Date.t(), interval()) :: Date.t()
+  def calculate_cycle_start(date, interval) do
+    calculate_cycle_start(date, interval, date)
+  end
+
+  @doc """
+  Calculates the end date of a cycle based on its start date and interval.
+
+  ## Parameters
+
+    - `cycle_start` - The start date of the cycle
+    - `interval` - The interval type
+
+  ## Returns
+
+    The end date of the cycle.
+
+  ## Examples
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_end(~D[2024-03-01], :monthly)
+      ~D[2024-03-31]
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_end(~D[2024-02-01], :monthly)
+      ~D[2024-02-29]
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_end(~D[2024-01-01], :quarterly)
+      ~D[2024-03-31]
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_end(~D[2024-01-01], :half_yearly)
+      ~D[2024-06-30]
+
+      iex> Mv.MembershipFees.CalendarCycles.calculate_cycle_end(~D[2024-01-01], :yearly)
+      ~D[2024-12-31]
+  """
+  @spec calculate_cycle_end(Date.t(), interval()) :: Date.t()
+  def calculate_cycle_end(cycle_start, interval) do
+    case interval do
+      :monthly -> monthly_cycle_end(cycle_start)
+      :quarterly -> quarterly_cycle_end(cycle_start)
+      :half_yearly -> half_yearly_cycle_end(cycle_start)
+      :yearly -> yearly_cycle_end(cycle_start)
+    end
+  end
+
+  @doc """
+  Calculates the start date of the next cycle.
+
+  ## Parameters
+
+    - `cycle_start` - The start date of the current cycle
+    - `interval` - The interval type
+
+  ## Returns
+
+    The start date of the next cycle.
+
+  ## Examples
+
+      iex> Mv.MembershipFees.CalendarCycles.next_cycle_start(~D[2024-01-01], :monthly)
+      ~D[2024-02-01]
+
+      iex> Mv.MembershipFees.CalendarCycles.next_cycle_start(~D[2024-01-01], :quarterly)
+      ~D[2024-04-01]
+
+      iex> Mv.MembershipFees.CalendarCycles.next_cycle_start(~D[2024-01-01], :half_yearly)
+      ~D[2024-07-01]
+
+      iex> Mv.MembershipFees.CalendarCycles.next_cycle_start(~D[2024-01-01], :yearly)
+      ~D[2025-01-01]
+  """
+  @spec next_cycle_start(Date.t(), interval()) :: Date.t()
+  def next_cycle_start(cycle_start, interval) do
+    cycle_end = calculate_cycle_end(cycle_start, interval)
+    next_date = Date.add(cycle_end, 1)
+    calculate_cycle_start(next_date, interval)
+  end
+
+  @doc """
+  Checks if the cycle contains the given date.
+
+  ## Parameters
+
+    - `cycle_start` - The start date of the cycle
+    - `interval` - The interval type
+    - `today` - The date to check (defaults to today's date)
+
+  ## Returns
+
+    `true` if the given date is within the cycle, `false` otherwise.
+
+  ## Examples
+
+      iex> Mv.MembershipFees.CalendarCycles.current_cycle?(~D[2024-03-01], :monthly, ~D[2024-03-15])
+      true
+
+      iex> Mv.MembershipFees.CalendarCycles.current_cycle?(~D[2024-02-01], :monthly, ~D[2024-03-15])
+      false
+
+      iex> Mv.MembershipFees.CalendarCycles.current_cycle?(~D[2024-03-01], :monthly, ~D[2024-03-01])
+      true
+
+      iex> Mv.MembershipFees.CalendarCycles.current_cycle?(~D[2024-03-01], :monthly, ~D[2024-03-31])
+      true
+  """
+  @spec current_cycle?(Date.t(), interval(), Date.t()) :: boolean()
+  def current_cycle?(cycle_start, interval, today) do
+    cycle_end = calculate_cycle_end(cycle_start, interval)
+
+    Date.compare(cycle_start, today) in [:lt, :eq] and
+      Date.compare(today, cycle_end) in [:lt, :eq]
+  end
+
+  @spec current_cycle?(Date.t(), interval()) :: boolean()
+  def current_cycle?(cycle_start, interval) do
+    current_cycle?(cycle_start, interval, Date.utc_today())
+  end
+
+  @doc """
+  Checks if the cycle is the last completed cycle.
+
+  A cycle is considered the last completed cycle if:
+  - The cycle has ended (cycle_end < today)
+  - The next cycle has not ended yet (today <= next_end)
+
+  In other words: `cycle_end < today <= next_end`
+
+  ## Parameters
+
+    - `cycle_start` - The start date of the cycle
+    - `interval` - The interval type
+    - `today` - The date to check against (defaults to today's date)
+
+  ## Returns
+
+    `true` if the cycle is the last completed cycle, `false` otherwise.
+
+  ## Examples
+
+      iex> Mv.MembershipFees.CalendarCycles.last_completed_cycle?(~D[2024-03-01], :monthly, ~D[2024-04-01])
+      true
+
+      iex> Mv.MembershipFees.CalendarCycles.last_completed_cycle?(~D[2024-03-01], :monthly, ~D[2024-03-15])
+      false
+
+      iex> Mv.MembershipFees.CalendarCycles.last_completed_cycle?(~D[2024-02-01], :monthly, ~D[2024-04-15])
+      false
+  """
+  @spec last_completed_cycle?(Date.t(), interval(), Date.t()) :: boolean()
+  def last_completed_cycle?(cycle_start, interval, today) do
+    cycle_end = calculate_cycle_end(cycle_start, interval)
+
+    # Cycle must have ended (cycle_end < today)
+    case Date.compare(today, cycle_end) do
+      :gt ->
+        # Check if this is the most recent completed cycle
+        # by verifying that the next cycle hasn't ended yet (today <= next_end)
+        next_start = next_cycle_start(cycle_start, interval)
+        next_end = calculate_cycle_end(next_start, interval)
+
+        Date.compare(today, next_end) in [:lt, :eq]
+
+      _ ->
+        false
+    end
+  end
+
+  @spec last_completed_cycle?(Date.t(), interval()) :: boolean()
+  def last_completed_cycle?(cycle_start, interval) do
+    last_completed_cycle?(cycle_start, interval, Date.utc_today())
+  end
+
+  # Private helper functions
+
+  defp monthly_cycle_start(date) do
+    Date.new!(date.year, date.month, 1)
+  end
+
+  defp monthly_cycle_end(cycle_start) do
+    Date.end_of_month(cycle_start)
+  end
+
+  defp quarterly_cycle_start(date) do
+    quarter_start_month =
+      case date.month do
+        m when m in [1, 2, 3] -> 1
+        m when m in [4, 5, 6] -> 4
+        m when m in [7, 8, 9] -> 7
+        m when m in [10, 11, 12] -> 10
+      end
+
+    Date.new!(date.year, quarter_start_month, 1)
+  end
+
+  defp quarterly_cycle_end(cycle_start) do
+    case cycle_start.month do
+      1 -> Date.new!(cycle_start.year, 3, 31)
+      4 -> Date.new!(cycle_start.year, 6, 30)
+      7 -> Date.new!(cycle_start.year, 9, 30)
+      10 -> Date.new!(cycle_start.year, 12, 31)
+    end
+  end
+
+  defp half_yearly_cycle_start(date) do
+    half_start_month = if date.month in 1..6, do: 1, else: 7
+    Date.new!(date.year, half_start_month, 1)
+  end
+
+  defp half_yearly_cycle_end(cycle_start) do
+    case cycle_start.month do
+      1 -> Date.new!(cycle_start.year, 6, 30)
+      7 -> Date.new!(cycle_start.year, 12, 31)
+    end
+  end
+
+  defp yearly_cycle_start(date) do
+    Date.new!(date.year, 1, 1)
+  end
+
+  defp yearly_cycle_end(cycle_start) do
+    Date.new!(cycle_start.year, 12, 31)
+  end
+end
diff --git a/test/mv/membership_fees/calendar_cycles_test.exs b/test/mv/membership_fees/calendar_cycles_test.exs
new file mode 100644
index 0000000..29fec48
--- /dev/null
+++ b/test/mv/membership_fees/calendar_cycles_test.exs
@@ -0,0 +1,181 @@
+defmodule Mv.MembershipFees.CalendarCyclesTest do
+  @moduledoc """
+  Tests for CalendarCycles module.
+  """
+  use ExUnit.Case, async: true
+
+  alias Mv.MembershipFees.CalendarCycles
+
+  doctest Mv.MembershipFees.CalendarCycles
+
+  describe "calculate_cycle_start/3" do
+    test "uses reference_date when provided" do
+      date = ~D[2024-03-15]
+      reference = ~D[2024-05-20]
+
+      assert CalendarCycles.calculate_cycle_start(date, :monthly, reference) == ~D[2024-05-01]
+      assert CalendarCycles.calculate_cycle_start(date, :quarterly, reference) == ~D[2024-04-01]
+    end
+  end
+
+  describe "current_cycle?/3" do
+    # Basic examples are covered by doctests
+
+    test "works for all interval types" do
+      today = ~D[2024-03-15]
+
+      for interval <- [:monthly, :quarterly, :half_yearly, :yearly] do
+        cycle_start = CalendarCycles.calculate_cycle_start(today, interval)
+        result = CalendarCycles.current_cycle?(cycle_start, interval, today)
+
+        assert result == true, "Expected current cycle for #{interval} with start #{cycle_start}"
+      end
+    end
+  end
+
+  describe "current_cycle?/2 wrapper" do
+    test "calls current_cycle?/3 with Date.utc_today()" do
+      today = Date.utc_today()
+      cycle_start = CalendarCycles.calculate_cycle_start(today, :monthly)
+
+      # This test verifies the wrapper works, but uses actual today
+      # The real testing happens in current_cycle?/3 tests above
+      result = CalendarCycles.current_cycle?(cycle_start, :monthly)
+
+      assert result == true
+    end
+  end
+
+  describe "last_completed_cycle?/3" do
+    # Basic examples are covered by doctests
+
+    test "returns false when next cycle has also ended" do
+      # Two cycles ago: cycle ended, but next cycle also ended
+      today = ~D[2024-05-15]
+      cycle_start = ~D[2024-03-01]
+      # Cycle ended 2024-03-31, next cycle ended 2024-04-30, today is 2024-05-15
+
+      assert CalendarCycles.last_completed_cycle?(cycle_start, :monthly, today) == false
+    end
+
+    test "works correctly for quarterly intervals" do
+      # Q1 2024 ended on 2024-03-31
+      # Q2 2024 ends on 2024-06-30
+      # Today is 2024-04-15 (after Q1 ended, before Q2 ended)
+      today = ~D[2024-04-15]
+      past_quarter_start = ~D[2024-01-01]
+
+      assert CalendarCycles.last_completed_cycle?(past_quarter_start, :quarterly, today) == true
+    end
+
+    test "returns false when cycle ended on the given date" do
+      # Cycle ends on today, so it's still current, not completed
+      today = ~D[2024-03-31]
+      cycle_start = ~D[2024-03-01]
+
+      assert CalendarCycles.last_completed_cycle?(cycle_start, :monthly, today) == false
+    end
+  end
+
+  describe "last_completed_cycle?/2 wrapper" do
+    test "calls last_completed_cycle?/3 with Date.utc_today()" do
+      today = Date.utc_today()
+      cycle_start = CalendarCycles.calculate_cycle_start(today, :monthly)
+
+      # This test verifies the wrapper works, but uses actual today
+      # The real testing happens in last_completed_cycle?/3 tests above
+      result = CalendarCycles.last_completed_cycle?(cycle_start, :monthly)
+
+      # Result depends on actual today, so we just verify it's a boolean
+      assert is_boolean(result)
+    end
+  end
+
+  describe "edge cases" do
+    test "leap year: February has 29 days" do
+      # 2024 is a leap year
+      assert CalendarCycles.calculate_cycle_end(~D[2024-02-01], :monthly) == ~D[2024-02-29]
+
+      # 2023 is not a leap year
+      assert CalendarCycles.calculate_cycle_end(~D[2023-02-01], :monthly) == ~D[2023-02-28]
+    end
+
+    test "year boundary: December 31 to January 1" do
+      # Yearly cycle
+      assert CalendarCycles.next_cycle_start(~D[2024-01-01], :yearly) == ~D[2025-01-01]
+
+      # Monthly cycle across year boundary
+      assert CalendarCycles.next_cycle_start(~D[2024-12-01], :monthly) == ~D[2025-01-01]
+
+      # Half-yearly cycle across year boundary
+      assert CalendarCycles.next_cycle_start(~D[2024-07-01], :half_yearly) == ~D[2025-01-01]
+
+      # Quarterly cycle across year boundary
+      assert CalendarCycles.next_cycle_start(~D[2024-10-01], :quarterly) == ~D[2025-01-01]
+    end
+
+    test "month boundary: different month lengths" do
+      # 31-day months
+      assert CalendarCycles.calculate_cycle_end(~D[2024-01-01], :monthly) == ~D[2024-01-31]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-03-01], :monthly) == ~D[2024-03-31]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-05-01], :monthly) == ~D[2024-05-31]
+
+      # 30-day months
+      assert CalendarCycles.calculate_cycle_end(~D[2024-04-01], :monthly) == ~D[2024-04-30]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-06-01], :monthly) == ~D[2024-06-30]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-09-01], :monthly) == ~D[2024-09-30]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-11-01], :monthly) == ~D[2024-11-30]
+    end
+
+    test "date in middle of cycle: all functions work correctly" do
+      middle_date = ~D[2024-03-15]
+
+      # calculate_cycle_start
+      assert CalendarCycles.calculate_cycle_start(middle_date, :monthly) == ~D[2024-03-01]
+      assert CalendarCycles.calculate_cycle_start(middle_date, :quarterly) == ~D[2024-01-01]
+      assert CalendarCycles.calculate_cycle_start(middle_date, :half_yearly) == ~D[2024-01-01]
+      assert CalendarCycles.calculate_cycle_start(middle_date, :yearly) == ~D[2024-01-01]
+
+      # calculate_cycle_end
+      monthly_start = CalendarCycles.calculate_cycle_start(middle_date, :monthly)
+      assert CalendarCycles.calculate_cycle_end(monthly_start, :monthly) == ~D[2024-03-31]
+
+      # next_cycle_start
+      assert CalendarCycles.next_cycle_start(monthly_start, :monthly) == ~D[2024-04-01]
+    end
+
+    test "quarterly: all quarter boundaries correct" do
+      # Q1 boundaries
+      assert CalendarCycles.calculate_cycle_start(~D[2024-01-15], :quarterly) == ~D[2024-01-01]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-01-01], :quarterly) == ~D[2024-03-31]
+
+      # Q2 boundaries
+      assert CalendarCycles.calculate_cycle_start(~D[2024-05-15], :quarterly) == ~D[2024-04-01]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-04-01], :quarterly) == ~D[2024-06-30]
+
+      # Q3 boundaries
+      assert CalendarCycles.calculate_cycle_start(~D[2024-08-15], :quarterly) == ~D[2024-07-01]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-07-01], :quarterly) == ~D[2024-09-30]
+
+      # Q4 boundaries
+      assert CalendarCycles.calculate_cycle_start(~D[2024-11-15], :quarterly) == ~D[2024-10-01]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-10-01], :quarterly) == ~D[2024-12-31]
+    end
+
+    test "half_yearly: both half boundaries correct" do
+      # First half boundaries
+      assert CalendarCycles.calculate_cycle_start(~D[2024-03-15], :half_yearly) == ~D[2024-01-01]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-01-01], :half_yearly) == ~D[2024-06-30]
+
+      # Second half boundaries
+      assert CalendarCycles.calculate_cycle_start(~D[2024-09-15], :half_yearly) == ~D[2024-07-01]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-07-01], :half_yearly) == ~D[2024-12-31]
+    end
+
+    test "yearly: full year boundaries" do
+      assert CalendarCycles.calculate_cycle_start(~D[2024-06-15], :yearly) == ~D[2024-01-01]
+      assert CalendarCycles.calculate_cycle_end(~D[2024-01-01], :yearly) == ~D[2024-12-31]
+      assert CalendarCycles.next_cycle_start(~D[2024-01-01], :yearly) == ~D[2025-01-01]
+    end
+  end
+end