fix(offset): correct ADT, AST, BST, GST and SST to GNU's values

benches/parse_datetime.rs

@@ -55,7 +55,16 @@ fn bench_relative_keywords(c: &mut Criterion) {
 }
 
 fn bench_weekday(c: &mut Criterion) {
-    c.bench_function("parse_weekday", |b| b.iter(|| parse_datetime("wednesday")));
+    // Use a fixed reference date: the cost of resolving a weekday depends on
+    // how many days separate it from "today", so relying on the current date
+    // makes the measurement vary with the day the benchmark runs.
+    // 2000-01-03 is a Monday, so "wednesday" resolves two days ahead.
+    let base = jiff::civil::DateTime::from(jiff::civil::date(2000, 1, 3))
+        .to_zoned(jiff::tz::TimeZone::UTC)
+        .unwrap();
+    c.bench_function("parse_weekday", |b| {
+        b.iter(|| parse_datetime_at_date(base.clone(), "wednesday"))
+    });
 }
 
 fn bench_timezone_offset(c: &mut Criterion) {

src/items/offset.rs

@@ -298,7 +298,7 @@ fn timezone_name_to_offset(input: &str) -> ModalResult<Offset> {
         "ut" => (false, 0, 0),
         "u" => (true, 8, 0),
         "t" => (true, 7, 0),
-        "sst" => (true, 11, 0),
+        "sst" => (true, 12, 0),
         "sgt" => (false, 8, 0),
         "sast" => (false, 2, 0),
         "s" => (true, 6, 0),
@@ -329,7 +329,7 @@ fn timezone_name_to_offset(input: &str) -> ModalResult<Offset> {
         "i" => (false, 9, 0),
         "hst" => (true, 10, 0),
         "h" => (false, 8, 0),
-        "gst" => (false, 4, 0),
+        "gst" => (false, 10, 0),
         "gmt" => (false, 0, 0),
         "g" => (false, 7, 0),
         "f" => (false, 6, 0),
@@ -348,15 +348,15 @@ fn timezone_name_to_offset(input: &str) -> ModalResult<Offset> {
         "cdt" => (true, 5, 0),
         "cat" => (false, 2, 0),
         "c" => (false, 3, 0),
-        "bst" => (false, 6, 0),
+        "bst" => (false, 1, 0),
         "brt" => (true, 3, 0),
         "brst" => (true, 2, 0),
         "b" => (false, 2, 0),
-        "ast" => (true, 3, 0),
+        "ast" => (true, 4, 0),
         "art" => (true, 3, 0),
         "akst" => (true, 9, 0),
         "akdt" => (true, 8, 0),
-        "adt" => (false, 4, 0),
+        "adt" => (true, 3, 0),
         "a" => (false, 1, 0),
         _ => return Err(ErrMode::Backtrack(ContextError::new())),
     };
@@ -459,6 +459,12 @@ mod tests {
             ("mesz", off(false, 2, 0)),
             ("mest", off(false, 2, 0)),
             ("kst", off(false, 9, 0)),
+            // Corrected to the offsets GNU date assigns these abbreviations.
+            ("adt", off(true, 3, 0)),   // Atlantic Daylight, was +4
+            ("ast", off(true, 4, 0)),   // Atlantic Standard, was -3
+            ("bst", off(false, 1, 0)),  // British Summer, was +6
+            ("gst", off(false, 10, 0)), // Guam Standard, was +4
+            ("sst", off(true, 12, 0)),  // was -11
             ("z123", off(false, 0, 0)), // space separator can be ignored if immediately followed by digits (GNU date behavior)
         ] {
             let mut s = input;

tests/date.rs

@@ -298,6 +298,19 @@ fn test_embedded_timezone(#[case] input: &str, #[case] expected: &str) {
     check_absolute(input, expected);
 }
 
+// Zone abbreviations must resolve to the offsets GNU date assigns them,
+// not to other real meanings of the same letters, e.g. BST as Bangladesh
+// Standard Time rather than British Summer Time.
+#[rstest]
+#[case::bst("2026-06-15 12:00 BST", "2026-06-15 12:00:00+01:00")]
+#[case::gst("2026-06-15 12:00 GST", "2026-06-15 12:00:00+10:00")]
+#[case::ast("2026-06-15 12:00 AST", "2026-06-15 12:00:00-04:00")]
+#[case::adt("2026-06-15 12:00 ADT", "2026-06-15 12:00:00-03:00")]
+#[case::sst("2026-06-15 12:00 SST", "2026-06-15 12:00:00-12:00")]
+fn test_zone_abbreviation_offsets(#[case] input: &str, #[case] expected: &str) {
+    check_absolute(input, expected);
+}
+
 // Regression test for uutils/coreutils#12555.
 // A fixed offset (e.g. the "UTC" keyword) must anchor the instant *before*
 // relative adjustments are applied. Otherwise, when the base zone observes DST,