Method for syncing interval functions over separate devices

I am trying to get an interval function, pre-programmed on separate devices, to sync up with each other. So several mobile devices all running the same interval function in sync. At first I thought I could just use the devices internal clocks and start the functions on the 0 of the minute. I realize now that mobile clocks aren't really all that accurate and are not synced to each other. Now I need a new solution.

I'm using heroku, node.js, socket.io, and ionic if that helps at all.