fix(gmail): honor configured timezone for sent mail dates

Co-authored-by: Dinakar Sarbada <dinakars777@users.noreply.github.com>

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Backup: add Gmail message-list checkpoints, streaming shard construction, and stderr progress counters so full-mailbox backups can resume cleanly after interruption without keeping every raw message in RAM.
 
 ### Fixed
+- Gmail: build outbound `Date` headers with the configured timezone so replies do not inherit a wrong host-local offset. (#514, #472) — thanks @dinakars777.
 - Gmail: preserve renewed watch expiration fields when a long-running `gmail watch serve` process records push delivery state after `gmail watch renew` runs separately. (#526)
 - Gmail: auto-fill draft reply subjects from the original message when `gmail drafts create --reply-to-message-id` omits `--subject`. (#488) — thanks @jbowerbir.
 - Drive: include `hasThumbnail` and `thumbnailLink` in `drive ls`, `drive search`, and `drive get` JSON responses. (#486) — thanks @gtapps.

internal/cmd/gmail_mime.go

@@ -86,7 +86,11 @@ func buildRFC822(opts mailOptions, cfg *rfc822Config) ([]byte, error) {
 		return nil, fmt.Errorf("invalid Subject: %w", err)
 	}
 	writeHeader(&b, "Subject", encodeHeaderIfNeeded(opts.Subject))
-	writeHeader(&b, "Date", time.Now().Format(time.RFC1123Z))
+	dateLocation, err := mailDateLocation()
+	if err != nil {
+		return nil, err
+	}
+	writeHeader(&b, "Date", time.Now().In(dateLocation).Format(time.RFC1123Z))
 	if !hasHeader(opts.AdditionalHeaders, "Message-ID") && !hasHeader(opts.AdditionalHeaders, "Message-Id") {
 		messageID, err := randomMessageID(opts.From)
 		if err != nil {
@@ -211,6 +215,17 @@ func buildRFC822(opts mailOptions, cfg *rfc822Config) ([]byte, error) {
 	return b.Bytes(), nil
 }
 
+func mailDateLocation() (*time.Location, error) {
+	loc, err := getConfiguredTimezone("")
+	if err != nil {
+		return nil, err
+	}
+	if loc != nil {
+		return loc, nil
+	}
+	return time.Local, nil
+}
+
 func writeHeader(b *bytes.Buffer, name, value string) {
 	b.WriteString(name)
 	b.WriteString(": ")

internal/cmd/gmail_mime_more_test.go

@@ -1,10 +1,12 @@
 package cmd
 
 import (
+	"net/mail"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestBuildRFC822_MissingFields(t *testing.T) {
@@ -33,6 +35,32 @@ func TestBuildRFC822_AllowMissingTo(t *testing.T) {
 	}
 }
 
+func TestBuildRFC822_DateHeaderUsesConfiguredTimezone(t *testing.T) {
+	oldLocal := time.Local
+	time.Local = time.FixedZone("JST", 9*60*60)
+	t.Cleanup(func() { time.Local = oldLocal })
+	t.Setenv("GOG_TIMEZONE", "UTC")
+
+	raw, err := buildRFC822(mailOptions{
+		From:    "a@b.com",
+		To:      []string{"c@d.com"},
+		Subject: "Hi",
+		Body:    "Hello",
+	}, nil)
+	if err != nil {
+		t.Fatalf("buildRFC822: %v", err)
+	}
+
+	msg, err := mail.ReadMessage(strings.NewReader(string(raw)))
+	if err != nil {
+		t.Fatalf("ReadMessage: %v", err)
+	}
+	date := msg.Header.Get("Date")
+	if !strings.HasSuffix(date, "+0000") {
+		t.Fatalf("expected UTC Date header, got %q", date)
+	}
+}
+
 func TestBuildRFC822_InvalidHeaders(t *testing.T) {
 	if _, err := buildRFC822(mailOptions{
 		From:    "a@b.com\r\nBcc: evil@evil.com",