Make sure you have a robust plugin layer as people will want to integrate this into their existing systems, like SCMs, issue trackers, CRMs, reporting and billing systems, chat clients (Teams/Slack) etc.
I love the idea, but I need this to work on my computer, and my phone, and my other computer, and..., so I'm heeding their call to build a cloud-based version of TagTime. It pings the user via various channels (SMS, Web Push, Slack, etc.) and feeds their responses into Beeminder, Toggl, and IFTTT.
I'd love to add TimeTagger to the list of supported integrations, since the demo app looks really compelling for the way I think about tracking time.
I run a service that tracks work/sleep/exercise/meditation (along with a couple other data points that don't fit the duration model) via an SMS interface.
I text it when I start/stop these activities and then it sends me reminders based on when I normally do those things during the day if I haven't already.
It would be great to be able to push the data I collect into your service to have a unified view of how I spend my time, and also to pull events from your service to send reminders to do certain things via my bot.