Add forecast daypart derivation

internal/timeutil/periods_test.go

@@ -0,0 +1,35 @@
+package timeutil
+
+import (
+	"testing"
+	"time"
+)
+
+func TestPeriodOverlapUsesHalfOpenIntervals(t *testing.T) {
+	start := time.Date(2026, 5, 29, 6, 0, 0, 0, time.UTC)
+	left := Period{Start: start, End: start.Add(time.Hour)}
+	touching := Period{Start: start.Add(time.Hour), End: start.Add(2 * time.Hour)}
+	overlapping := Period{Start: start.Add(30 * time.Minute), End: start.Add(90 * time.Minute)}
+
+	if left.Overlaps(touching) {
+		t.Fatal("touching half-open periods overlap, want false")
+	}
+	if !left.Overlaps(overlapping) {
+		t.Fatal("overlapping periods do not overlap, want true")
+	}
+}
+
+func TestClockWindowHandlesOvernight(t *testing.T) {
+	location := time.FixedZone("Test", -5*60*60)
+	date := time.Date(2026, 5, 29, 12, 0, 0, 0, location)
+	start, _ := ParseClock("22:00")
+	end, _ := ParseClock("06:00")
+
+	window := ClockWindow(date, location, start, end)
+	if got := window.Start.Format("2006-01-02T15:04"); got != "2026-05-29T22:00" {
+		t.Fatalf("Start = %s, want 2026-05-29T22:00", got)
+	}
+	if got := window.End.Format("2006-01-02T15:04"); got != "2026-05-30T06:00" {
+		t.Fatalf("End = %s, want 2026-05-30T06:00", got)
+	}
+}