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AchoArnoldCopilot
AchoArnold
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Copilot
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feat(billing): add ComputeBillingCycle function with dynamic clamping
Implement billing cycle computation that: - Accepts a timestamp and anchor day (1-31) - Dynamically clamps anchor day to month's actual days - Handles edge cases: February short months, leap years, year boundaries - Returns (start, end) tuple representing full billing cycle window Includes comprehensive test suite with 9 test cases covering: - Calendar month alignment (anchor=1) - Mid-month anchors (anchor=15) - Edge cases (leap years, month boundaries) - Year boundaries All tests pass. Implementation is tested and verified. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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‎api/pkg/entities/billing_cycle.go‎

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package entities
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import "time"
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// ComputeBillingCycle returns the start and end timestamps of the billing cycle
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// that contains `now`, given the user's anchor day (1–31). The anchor day is
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// dynamically clamped to the number of days in the relevant month.
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func ComputeBillingCycle(now time.Time, anchorDay int) (start, end time.Time) {
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clampedDay := min(anchorDay, daysInMonth(now.Year(), now.Month()))
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if now.Day() >= clampedDay {
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// Cycle started this month
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start = time.Date(now.Year(), now.Month(), clampedDay, 0, 0, 0, 0, time.UTC)
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} else {
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// Cycle started last month
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prev := now.AddDate(0, -1, 0)
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prevClamped := min(anchorDay, daysInMonth(prev.Year(), prev.Month()))
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start = time.Date(prev.Year(), prev.Month(), prevClamped, 0, 0, 0, 0, time.UTC)
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}
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// Compute next cycle start by moving to next month and clamping the day
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nextMonth := start.Month() + 1
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nextYear := start.Year()
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if nextMonth > 12 {
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nextMonth = 1
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nextYear++
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}
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nextClamped := min(anchorDay, daysInMonth(nextYear, nextMonth))
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nextCycleStart := time.Date(nextYear, nextMonth, nextClamped, 0, 0, 0, 0, time.UTC)
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// End = one second before the next cycle start
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end = nextCycleStart.Add(-time.Second)
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return start, end
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}
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// daysInMonth returns the number of days in the given month/year.
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func daysInMonth(year int, month time.Month) int {
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return time.Date(year, month+1, 0, 0, 0, 0, 0, time.UTC).Day()
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}

‎api/pkg/entities/billing_cycle_test.go‎

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package entities
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import (
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"testing"
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"time"
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"github.com/stretchr/testify/assert"
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)
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func TestComputeBillingCycle(t *testing.T) {
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tests := []struct {
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name string
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now time.Time
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anchorDay int
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wantStart time.Time
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wantEnd time.Time
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}{
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{
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name: "anchor day 1 (same as calendar month)",
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now: time.Date(2026, 5, 15, 10, 0, 0, 0, time.UTC),
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anchorDay: 1,
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wantStart: time.Date(2026, 5, 1, 0, 0, 0, 0, time.UTC),
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wantEnd: time.Date(2026, 5, 31, 23, 59, 59, 0, time.UTC),
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},
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{
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name: "anchor day 15, now is after anchor",
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now: time.Date(2026, 5, 20, 10, 0, 0, 0, time.UTC),
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anchorDay: 15,
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wantStart: time.Date(2026, 5, 15, 0, 0, 0, 0, time.UTC),
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wantEnd: time.Date(2026, 6, 14, 23, 59, 59, 0, time.UTC),
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},
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{
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name: "anchor day 15, now is before anchor",
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now: time.Date(2026, 5, 10, 10, 0, 0, 0, time.UTC),
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anchorDay: 15,
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wantStart: time.Date(2026, 4, 15, 0, 0, 0, 0, time.UTC),
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wantEnd: time.Date(2026, 5, 14, 23, 59, 59, 0, time.UTC),
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},
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{
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name: "anchor day 15, now is exactly on anchor",
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now: time.Date(2026, 5, 15, 0, 0, 0, 0, time.UTC),
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anchorDay: 15,
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wantStart: time.Date(2026, 5, 15, 0, 0, 0, 0, time.UTC),
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wantEnd: time.Date(2026, 6, 14, 23, 59, 59, 0, time.UTC),
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},
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{
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name: "anchor day 31 in February (clamped to 28)",
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now: time.Date(2026, 2, 15, 10, 0, 0, 0, time.UTC),
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anchorDay: 31,
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wantStart: time.Date(2026, 1, 31, 0, 0, 0, 0, time.UTC),
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wantEnd: time.Date(2026, 2, 27, 23, 59, 59, 0, time.UTC),
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},
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{
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name: "anchor day 31 in March (not clamped)",
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now: time.Date(2026, 3, 31, 10, 0, 0, 0, time.UTC),
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anchorDay: 31,
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wantStart: time.Date(2026, 3, 31, 0, 0, 0, 0, time.UTC),
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wantEnd: time.Date(2026, 4, 29, 23, 59, 59, 0, time.UTC),
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},
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{
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name: "anchor day 29 in February leap year",
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now: time.Date(2024, 2, 29, 10, 0, 0, 0, time.UTC),
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anchorDay: 29,
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wantStart: time.Date(2024, 2, 29, 0, 0, 0, 0, time.UTC),
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wantEnd: time.Date(2024, 3, 28, 23, 59, 59, 0, time.UTC),
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},
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{
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name: "anchor day 29 in February non-leap year (clamped to 28)",
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now: time.Date(2026, 2, 28, 10, 0, 0, 0, time.UTC),
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anchorDay: 29,
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wantStart: time.Date(2026, 2, 28, 0, 0, 0, 0, time.UTC),
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wantEnd: time.Date(2026, 3, 28, 23, 59, 59, 0, time.UTC),
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},
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{
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name: "year boundary: anchor day 20, now is Jan 5",
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now: time.Date(2026, 1, 5, 10, 0, 0, 0, time.UTC),
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anchorDay: 20,
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wantStart: time.Date(2025, 12, 20, 0, 0, 0, 0, time.UTC),
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wantEnd: time.Date(2026, 1, 19, 23, 59, 59, 0, time.UTC),
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},
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}
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for _, tt := range tests {
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t.Run(tt.name, func(t *testing.T) {
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start, end := ComputeBillingCycle(tt.now, tt.anchorDay)
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assert.Equal(t, tt.wantStart, start)
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assert.Equal(t, tt.wantEnd, end)
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})
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}
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}
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func TestDaysInMonth(t *testing.T) {
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assert.Equal(t, 31, daysInMonth(2026, time.January))
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assert.Equal(t, 28, daysInMonth(2026, time.February))
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assert.Equal(t, 29, daysInMonth(2024, time.February))
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assert.Equal(t, 30, daysInMonth(2026, time.April))
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assert.Equal(t, 31, daysInMonth(2026, time.December))
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}

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