As already mentioned in some other posts, like this one:
http://help.doit.im/topics/628

If you have a task scheduled for today with a specific time and move it to another date (for example tomorrow) but don't want to change the time, you cannot save. You have to change the time and then change it back to activate the save-button.

As I use this feature every day I find it REALLY annoying. And I'm not the only one.

This should be an easy one to fix. Could you please do so in the next release?

TIA.