At the moment, when you call server.sleepfor the device logging in the IDE displays something like the following:
2016-09-29 16:42:10 UTC+1 [Device] sleeping until 1475163760000
That’s the unix timestamp (in milliseconds) when the device expects to wake up. It’s basically unreadable.
In the next production deploy (whenever that is…), this will change to display the time in ISO 8601 (ish – there’s no ‘T’ delimiter):
2016-09-29 18:54:45 UTC+1 [Device] Sleeping until 2016-09-29 17:55:15Z
So, if you’re parsing these strings directly out of the API (and we don’t support that, so don’t do it), be aware that the format has changed.