fix: resolve dates and bucket timestamps in local time (#14)

Two bug classes were mixing UTC and local time across the CLI:

1. Absolute date flags (--since 2025-01-01, show 2025-03-08) used
   time.Parse which defaults to UTC. In EDT that's Jan 1 midnight UTC =
   Dec 31 8pm local, so --since silently included yesterday evening's
   data and show <date> returned workouts from the wrong calendar day.
   Relative flags (--since 7d, today, yesterday) already anchored to
   local time, so the two forms of the same flag had different
   semantics.

2. API timestamps (StartedAt, RFC3339 with the server's zone) were
   formatted to YYYY-MM-DD without converting to local first. A 11pm
   local workout (4am UTC next day) bucketed under the wrong date in
   stats and monthly bodyweight averages, and failed to match in
   'workouts show <date>'.

Fix is mechanical: ParseInLocation(..., time.Local) for user date
parsing, and .Local() before every Format("2006-01-02") used as a
bucket key or display string.

Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>

Author: DTTerastar

cmd/bodyweights.go

@@ -105,6 +105,7 @@ func loadBodyweightEntries(sinceFlag string) ([]bodyweightEntry, error) {
 		if err != nil {
 			continue
 		}
+		date = date.Local()
 		if !since.IsZero() && date.Before(since) {
 			continue
 		}

cmd/stats.go

@@ -103,7 +103,7 @@ func buildSummaries(posts []Post) []ExerciseSummary {
 	for _, p := range posts {
 		bw, _ := strconv.ParseFloat(strings.TrimSpace(p.Bodyweight), 64)
 		t, _ := time.Parse(time.RFC3339Nano, p.StartedAt)
-		date := t.Format("2006-01-02")
+		date := t.Local().Format("2006-01-02")
 
 		for _, e := range p.ExerciseData {
 			ss := sessionStats(e, bw)

cmd/workouts.go

@@ -87,7 +87,7 @@ var listCmd = &cobra.Command{
 
 func parseSince(s string) (time.Time, error) {
 	// Try absolute date first
-	if t, err := time.Parse("2006-01-02", s); err == nil {
+	if t, err := time.ParseInLocation("2006-01-02", s, time.Local); err == nil {
 		return t, nil
 	}
 	// Try relative: e.g. 30d, 4w, 6m, 1y
@@ -137,7 +137,7 @@ var showCmd = &cobra.Command{
 			if err != nil {
 				continue
 			}
-			if t.Format("2006-01-02") == target.Format("2006-01-02") {
+			if t.Local().Format("2006-01-02") == target.Format("2006-01-02") {
 				matched = append(matched, p)
 			}
 		}
@@ -162,7 +162,7 @@ func parseDate(s string) (time.Time, error) {
 	case "yesterday":
 		return now.AddDate(0, 0, -1), nil
 	}
-	if t, err := time.Parse("2006-01-02", s); err == nil {
+	if t, err := time.ParseInLocation("2006-01-02", s, time.Local); err == nil {
 		return t, nil
 	}
 	return time.Time{}, fmt.Errorf("invalid date: %q (use YYYY-MM-DD, today, or yesterday)", s)
@@ -223,7 +223,7 @@ func printFitdown(posts []Post) error {
 		if err != nil {
 			fmt.Printf("Workout %s\n", post.StartedAt)
 		} else {
-			fmt.Printf("Workout %s\n", t.Format("January 2, 2006"))
+			fmt.Printf("Workout %s\n", t.Local().Format("January 2, 2006"))
 		}
 
 		if post.SessionNotes != "" {