Hi,
There are a few ways to handle the date-time in the header. Basically the options come down to changing the format used by the tag used to generate that date-time or altering the default format used by date-time tags in all reports or just the Seasons skin. I suspect you probably want the former.
To change the format used by the tag used to generate that date-time you need to edit skins/Seasons/titlebar.inc and change the following line from (untested):
<p class="lastupdate">$current.dateTime</p>
to:
<p class="lastupdate">$current.dateTime.format(format_string="%Y-%m-%d %H:%M")</p>
Save the file and the change should occur on the next report cycle. For info, the available format codes are listed in the table
here.
For the date-time formats used in the plot labels you need to alter settings under the [ImageGenerator] stanza in the Season skin config file (skins/Seasons/skin.conf), specifically the bottom_label_format setting. The ImageGenerator label options are covered here in the Customization Guide. The format codes used by bottom_label_format are the same as linked earlier in this post. Note also that bottom_label_format is set for each set (day, week, month, year) of plots.
The above changes should be retained across WeeWX upgrades but you may wish to make note of your changes should you need to restore them in the future.