I have just tried to define a new rule.
When I select 'Time Of Day' and then 'Between Two Times', if I select a specific time then the time input does not save. If I select either sunset or sunrise then that is saved OK. If I leave it blank it automatically inputs the current time.
I have tried in Chrome, (normal and incognito) and Firefox.
Same result.
There is a bug in entering time in 24 hour format, which is probably what you're doing. Temporarily switch to 12 hour, setup the rule, then switch back. But has been fixed, in next release.
Glad I stumbled on this thread. I was having the same problem with automating my Christmas Lights turn-on/off. Since sunrise/sunset worked, I just said "sunset + 360 minutes" for the turn-off time.
I'll redo my rules now (or leave them... but at least I know how to fix).