fix: TZ prefix should override base date timezone

src/items/builder.rs

@@ -187,9 +187,13 @@ impl DateTimeBuilder {
     ///   - e. Apply final fixed offset if present.
     pub(super) fn build(self) -> Result<Zoned, error::Error> {
         // 1. Choose the base instant.
-        let base = match (self.base, &self.timezone) {
-            (Some(b), _) => b,
-            (None, Some(tz)) => jiff::Timestamp::now().to_zoned(tz.clone()),
+        // If a TZ="..." prefix was parsed, it should override the base's timezone
+        // while keeping the base's timestamp for relative date calculations.
+        let has_timezone = self.timezone.is_some();
+        let base = match (self.base, self.timezone) {
+            (Some(b), Some(tz)) => b.timestamp().to_zoned(tz),
+            (Some(b), None) => b,
+            (None, Some(tz)) => jiff::Timestamp::now().to_zoned(tz),
             (None, None) => Zoned::now(),
         };
 
@@ -204,7 +208,7 @@ impl DateTimeBuilder {
             || self.time.is_some()
             || self.weekday.is_some()
             || self.offset.is_some()
-            || self.timezone.is_some();
+            || has_timezone;
 
         let mut dt = if need_midnight {
             base.with().time(civil::time(0, 0, 0, 0)).build()?

tests/date.rs

@@ -95,3 +95,20 @@ fn test_date_omitting_year(#[case] input: &str, #[case] year: u32, #[case] expec
         .unwrap();
     check_relative(now, input, expected);
 }
+
+#[rstest]
+#[case::tz_prefix_est5("TZ=\"EST5\" 1970-01-01 00:00", "1970-01-01 00:00:00-05:00")]
+#[case::tz_prefix_pst8("TZ=\"PST8\" 1970-01-01 00:00", "1970-01-01 00:00:00-08:00")]
+#[case::tz_prefix_utc("TZ=\"UTC\" 1970-01-01 12:00", "1970-01-01 12:00:00+00:00")]
+#[case::tz_prefix_europe_paris(
+    r#"TZ="Europe/Paris" 2025-01-02 03:04:05"#,
+    "2025-01-02 03:04:05+01:00"
+)]
+fn test_tz_prefix_with_base_date(#[case] input: &str, #[case] expected: &str) {
+    let base = "2020-06-15 12:00:00"
+        .parse::<DateTime>()
+        .unwrap()
+        .to_zoned(TimeZone::UTC)
+        .unwrap();
+    check_relative(base, input, expected);
+}