If the date-min/max config value is a string, try parsing it using
datetime.strptime [1] with 'date-format' as format string [2]
(default: "%Y-%m-%dT%H:%M:%S")
Example: get all submissions posted in 2016
$ gallery-dl reddit.com/r/... \
-o date-format=%Y \
-o date-min=\"2016\" \
-o date-max=\"2017\"
[1] https://docs.python.org/3/library/datetime.html#datetime.datetime.strptime
[2] https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior