Flexible Desktop Timer.
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Pro Light version available
There is a cheaper light version available for those that don't need the full functionality of the app or would like to try it out without spending the more money on the more expensive version.
Pro Export time to CSV
Users can export CSV files from the app to use for billing, etc. Though this does require the non light version.
Pro An internet connection isn't required
Due to the app having no cloud sync there is no need to have an internet connection for the app to run. Everything it does is local to the device.
Pro Sync time across devices using a shared folder
While not necessarily the easiest of options (really more a work around to the apps limited functionality) users can sync the apps files to a shared folder or cloud storage folder in order to have sync'd time across Mac's.
Con Only available for OSX
Can only be used on OSX devices, which is pretty limiting for those that use multiple OS's.