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
+}