When downloading with the timestamp option (-R, --remote-time true) the modified date of the downloaded file is exactly one hour ahead when the server file has a modified date that's within my tz's DST range.

At least, that's my finding comparing downloading files out- and inside the DST range.
Files outside the DST range seem to get correctly timestamped.

So e.g. a file with a Last-Modified datetime of Mon, 07 Aug 2017 10:23:00 GMT which Windows displays correctly as 12:23:00 otherwise (tz = GMT+1), becomes 13:23:00 when downloaded with Aria2.

Wget seems to suffer from the same flaw. Curl doesn't.

This is all on Windows (7) btw. Wget on a CentOS VM doesn't seem to suffer from this flaw.
Haven't tried to compile Aria2 to test this as well inside the VM.