fix unsigned to signed conversion bug (#656)

time/src/sys/local_offset_at/windows.rs

@@ -59,10 +59,9 @@ fn systemtime_to_filetime(systime: &SystemTime) -> Option<FileTime> {
 /// Convert a `FILETIME` to an `i64`, representing a number of seconds.
 fn filetime_to_secs(filetime: &FileTime) -> i64 {
     /// FILETIME represents 100-nanosecond intervals
-    const FT_TO_SECS: i64 = Nanosecond::per(Second) as i64 / 100;
-    (filetime.dwHighDateTime.cast_signed().extend::<i64>() << 32
-        | filetime.dwLowDateTime.cast_signed().extend::<i64>())
-        / FT_TO_SECS
+    const FT_TO_SECS: u64 = Nanosecond::per(Second) as u64 / 100;
+    ((filetime.dwHighDateTime.extend::<u64>() << 32 | filetime.dwLowDateTime.extend::<u64>())
+        / FT_TO_SECS) as i64
 }
 
 /// Convert an [`OffsetDateTime`] to a `SYSTEMTIME`.