Addon for Chrome and Firefox that inserts a time tracking button on the issue page.
See gitlab-ce#27824, which is as of yet (september 2017) still open.
First, you need a personal access token from any instance you want to be connected to. Add the instance and your access token on the addon's settings page. You need to grant the addon permission for that site.
On any issue page, press 'start' to start the timer. You can pause it afterwards, or use 'save' to submit the time to the given issue. Use 'trash' to discard any accrued time. You can have multiple timers (running in parallel.) Timers persist across browser sessions.