GitLab defaults its time zone to UTC, which may not be what you want. Thankfully, you can update the value directly from your gitlab.rb file. Here's the relevant line:

gitlab_rails['time_zone'] = 'America/New_York'

Once you've added the field, simply reconfigure and restart:

sudo gitlab-ctl reconfigure
sudo gitlab-ctl restart

A list of all the available timezones is available on Wikipedia.