Correct offset calculation on wasm_bindgen

src/offset/local/mod.rs

@@ -50,15 +50,34 @@ mod inner {
     not(any(target_os = "emscripten", target_os = "wasi"))
 ))]
 mod inner {
-    use crate::{FixedOffset, LocalResult, NaiveDateTime};
+    use crate::{Datelike, FixedOffset, LocalResult, NaiveDateTime, Timelike};
 
-    pub(super) fn offset_from_utc_datetime(_utc: &NaiveDateTime) -> LocalResult<FixedOffset> {
-        let offset = js_sys::Date::new_0().get_timezone_offset();
+    pub(super) fn offset_from_utc_datetime(utc: &NaiveDateTime) -> LocalResult<FixedOffset> {
+        let offset = js_sys::Date::from(utc.and_utc()).get_timezone_offset();
         LocalResult::Single(FixedOffset::west_opt((offset as i32) * 60).unwrap())
     }
 
     pub(super) fn offset_from_local_datetime(local: &NaiveDateTime) -> LocalResult<FixedOffset> {
-        offset_from_utc_datetime(local)
+        let mut year = local.year();
+        if year < 100 {
+            // The API in `js_sys` does not let us create a `Date` with negative years.
+            // And values for years from `0` to `99` map to the years `1900` to `1999`.
+            // Shift the value by a multiple of 400 years until it is `>= 100`.
+            let shift_cycles = (year - 100).div_euclid(400);
+            year -= shift_cycles * 400;
+        }
+        let js_date = js_sys::Date::new_with_year_month_day_hr_min_sec(
+            year as u32,
+            local.month0() as i32,
+            local.day() as i32,
+            local.hour() as i32,
+            local.minute() as i32,
+            local.second() as i32,
+            // ignore milliseconds, our representation of leap seconds may be problematic
+        );
+        let offset = js_date.get_timezone_offset();
+        // We always get a result, even if this time does not exist or is ambiguous.
+        LocalResult::Single(FixedOffset::west_opt((offset as i32) * 60).unwrap())
     }
 }