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.