fix: Google Tasks due dates shifted a day earlier in negative-offset timezones

Google Tasks' due field is date-only but always serialized as an
RFC3339 timestamp fixed at midnight UTC regardless of the user's
timezone. Converting that instant into displayTZ (dueDate.In(tz))
reinterpreted it as a real moment in time instead of re-anchoring the
same calendar date to local midnight -- midnight UTC becomes 2pm the
previous day in UTC-10 (Pacific/Honolulu), so every dated Google Task
silently landed one day earlier than its actual due date. A task due
tomorrow showed up in today's section.

Fix: extract the Y/M/D from the UTC-anchored timestamp (which IS the
intended calendar date) and rebuild midnight in displayTZ from those
components, instead of converting the instant.

1 files changed, 13 insertions, 2 deletions

diff --git a/internal/api/google_tasks.go b/internal/api/google_tasks.go
index b580be4..1f9aefb 100644
--- a/internal/api/google_tasks.go
+++ b/internal/api/google_tasks.go
@@ -95,14 +95,25 @@ func (c *GoogleTasksClient) getTasksFromList(ctx context.Context, listID string)
 
 		// Parse due date if present (RFC3339 format, but date-only)
 		if item.Due != "" {
-			// Google Tasks due dates are in RFC3339 format but typically just the date part
+			// Google Tasks due dates are date-only, but the API always
+			// serializes them as an RFC3339 timestamp fixed at midnight UTC
+			// regardless of the user's timezone. Converting that instant
+			// into displayTZ (dueDate.In(c.displayTZ)) reinterprets it as a
+			// real moment in time and shifts the calendar date backward for
+			// any timezone behind UTC -- midnight UTC becomes 2pm the
+			// PREVIOUS day in UTC-10, silently moving every dated Google
+			// Task one day earlier than its actual due date. The Y/M/D of
+			// the UTC-anchored timestamp IS the intended calendar date, so
+			// re-anchor those same components to displayTZ midnight instead
+			// of converting the instant.
 			dueDate, err := time.Parse(time.RFC3339, item.Due)
 			if err != nil {
 				// Try date-only format
 				dueDate, err = time.ParseInLocation("2006-01-02", item.Due[:10], c.displayTZ)
 			}
 			if err == nil {
-				dueInTZ := dueDate.In(c.displayTZ)
+				y, m, d := dueDate.Date()
+				dueInTZ := time.Date(y, m, d, 0, 0, 0, 0, c.displayTZ)
 				task.DueDate = &dueInTZ
 			}
 		}