Initial commit

internal/daterange/daterange.go

@@ -0,0 +1,46 @@
+// Package daterange resolves recurring calendar windows and normalized dates.
+package daterange
+
+import "time"
+
+// CalendarWindow resolves a recurring month/day range around now. A range whose
+// start month is after its end month crosses the year boundary.
+func CalendarWindow(now time.Time, monthFrom int, dayFrom *int, monthTo int, dayTo *int) (time.Time, time.Time) {
+	location := now.Location()
+	startYear := now.Year()
+	if monthFrom > monthTo && int(now.Month()) <= monthTo {
+		startYear--
+	}
+	endYear := startYear
+	if monthFrom > monthTo {
+		endYear++
+	}
+	startDay := 1
+	if dayFrom != nil {
+		startDay = clampDay(startYear, time.Month(monthFrom), *dayFrom)
+	}
+	endDay := daysInMonth(endYear, time.Month(monthTo))
+	if dayTo != nil {
+		endDay = clampDay(endYear, time.Month(monthTo), *dayTo)
+	}
+	return time.Date(startYear, time.Month(monthFrom), startDay, 0, 0, 0, 0, location), time.Date(endYear, time.Month(monthTo), endDay, 23, 59, 59, 0, location)
+}
+
+// Date returns value at midnight in its original location.
+func Date(value time.Time) time.Time {
+	return time.Date(value.Year(), value.Month(), value.Day(), 0, 0, 0, 0, value.Location())
+}
+
+func daysInMonth(year int, month time.Month) int {
+	return time.Date(year, month+1, 0, 0, 0, 0, 0, time.UTC).Day()
+}
+
+func clampDay(year int, month time.Month, day int) int {
+	if maximum := daysInMonth(year, month); day > maximum {
+		return maximum
+	}
+	if day < 1 {
+		return 1
+	}
+	return day
+}

internal/daterange/daterange_test.go

@@ -0,0 +1,36 @@
+package daterange
+
+import (
+	"testing"
+	"time"
+)
+
+func intPointer(value int) *int { return &value }
+
+func TestCalendarWindow(t *testing.T) {
+	tests := []struct {
+		name               string
+		now                time.Time
+		from               int
+		fromDay            *int
+		to                 int
+		toDay              *int
+		wantStart, wantEnd string
+	}{
+		{"ordinary", time.Date(2026, 6, 1, 0, 0, 0, 0, time.UTC), 2, nil, 3, nil, "2026-02-01", "2026-03-31"},
+		{"wrap before end", time.Date(2026, 1, 10, 0, 0, 0, 0, time.UTC), 11, nil, 2, nil, "2025-11-01", "2026-02-28"},
+		{"wrap before start", time.Date(2026, 9, 1, 0, 0, 0, 0, time.UTC), 11, nil, 2, nil, "2026-11-01", "2027-02-28"},
+		{"clamps invalid month day", time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC), 2, intPointer(31), 2, intPointer(31), "2026-02-28", "2026-02-28"},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			start, end := CalendarWindow(test.now, test.from, test.fromDay, test.to, test.toDay)
+			if got := start.Format("2006-01-02"); got != test.wantStart {
+				t.Errorf("start=%s", got)
+			}
+			if got := end.Format("2006-01-02"); got != test.wantEnd {
+				t.Errorf("end=%s", got)
+			}
+		})
+	}
+}