[postprocessor:mtime] fix timestamps from datetime objects (#2307)

'datetime.timestamp()', which got used to convert datetime objects to
POSIX timestamps, assumes naive datetimes represent LOCAL time, while
datetimes in 'date' metadata fields represent UTC time.

Ref: https://docs.python.org/3/library/datetime.html#datetime.datetime.timestamp
> Naive datetime instances are assumed to represent local time
> you can obtain the POSIX timestamp by … calculating the timestamp directly

gallery_dl/postprocessor/mtime.py

@@ -9,7 +9,8 @@
 """Use metadata as file modification time"""
 
 from .common import PostProcessor
-from ..text import parse_int
+from .. import text, util
+from datetime import datetime
 
 
 class MtimePP(PostProcessor):
@@ -27,8 +28,11 @@ class MtimePP(PostProcessor):
 
     def run(self, pathfmt):
         mtime = pathfmt.kwdict.get(self.key)
-        ts = getattr(mtime, "timestamp", None)
-        pathfmt.kwdict["_mtime"] = ts() if ts else parse_int(mtime)
+        pathfmt.kwdict["_mtime"] = (
+            util.datetime_to_timestamp(mtime)
+            if isinstance(mtime, datetime) else
+            text.parse_int(mtime)
+        )
 
 
 __postprocessor__ = MtimePP