Fix dashboard date bucketing

internal/dashboard/period.go

@@ -6,7 +6,7 @@ import (
 )
 
 func PeriodWindows(eventTime time.Time, statZone *time.Location) []PeriodWindow {
-	localDate := startOfDay(eventTime.In(statZone))
+	localDate := startOfDay(wallClockInLocation(eventTime, statZone))
 	dayEnd := localDate.AddDate(0, 0, 1)
 
 	weekday := int(localDate.Weekday())
@@ -59,8 +59,8 @@ func SamePeriod(left time.Time, right time.Time, window PeriodWindow, statZone *
 	if window.Type == PeriodAll {
 		return true
 	}
-	leftLocal := left.In(statZone)
-	rightLocal := right.In(statZone)
+	leftLocal := wallClockInLocation(left, statZone)
+	rightLocal := wallClockInLocation(right, statZone)
 	switch window.Type {
 	case PeriodDay:
 		return startOfDay(leftLocal).Equal(startOfDay(rightLocal))
@@ -76,7 +76,7 @@ func SamePeriod(left time.Time, right time.Time, window PeriodWindow, statZone *
 }
 
 func DailyDate(eventTime time.Time, storageZone *time.Location) time.Time {
-	return startOfDay(eventTime.In(storageZone))
+	return startOfDay(wallClockInLocation(eventTime, storageZone))
 }
 
 func DateNumber(day time.Time) int {
@@ -93,3 +93,15 @@ func startOfDay(value time.Time) time.Time {
 	year, month, date := value.Date()
 	return time.Date(year, month, date, 0, 0, 0, 0, value.Location())
 }
+
+func wallClockInLocation(value time.Time, location *time.Location) time.Time {
+	if value.IsZero() {
+		return value
+	}
+	if location == nil {
+		location = time.UTC
+	}
+	year, month, day := value.Date()
+	hour, minute, second := value.Clock()
+	return time.Date(year, month, day, hour, minute, second, value.Nanosecond(), location)
+}

internal/dashboard/period_test.go

@@ -0,0 +1,50 @@
+package dashboard
+
+import (
+	"testing"
+	"time"
+)
+
+func TestDailyDateKeepsDatabaseDatetimeCalendarDay(t *testing.T) {
+	riyadh := mustLoadLocation(t, "Asia/Riyadh")
+	eventTime := time.Date(2026, 5, 26, 1, 30, 0, 0, time.UTC)
+
+	got := DailyDate(eventTime, riyadh)
+
+	if got.Format("2006-01-02") != "2026-05-26" {
+		t.Fatalf("DailyDate shifted database DATETIME calendar day: got %s", got.Format("2006-01-02"))
+	}
+	if got.Location() != riyadh {
+		t.Fatalf("DailyDate location = %s, want %s", got.Location(), riyadh)
+	}
+}
+
+func TestPeriodWindowsKeepsDatabaseDatetimeCalendarDay(t *testing.T) {
+	riyadh := mustLoadLocation(t, "Asia/Riyadh")
+	eventTime := time.Date(2026, 5, 26, 1, 30, 0, 0, time.UTC)
+
+	windows := PeriodWindows(eventTime, riyadh)
+
+	if windows[0].Type != PeriodDay || windows[0].Key != "2026-05-26" {
+		t.Fatalf("day window = %+v, want key 2026-05-26", windows[0])
+	}
+}
+
+func TestSamePeriodKeepsDatabaseDatetimeCalendarDay(t *testing.T) {
+	riyadh := mustLoadLocation(t, "Asia/Riyadh")
+	left := time.Date(2026, 5, 26, 0, 10, 0, 0, time.UTC)
+	right := time.Date(2026, 5, 26, 23, 50, 0, 0, time.UTC)
+
+	if !SamePeriod(left, right, PeriodWindow{Type: PeriodDay}, riyadh) {
+		t.Fatal("SamePeriod shifted database DATETIME values out of the same calendar day")
+	}
+}
+
+func mustLoadLocation(t *testing.T, name string) *time.Location {
+	t.Helper()
+	location, err := time.LoadLocation(name)
+	if err != nil {
+		t.Fatalf("load location %s: %v", name, err)
+	}
+	return location
+}