Allow selection of time at every 15 or 30 minutes interval

[sabbiu]

Is it possible to restrict the time selection at only 15 minutes interval?

So that user can select following time,

02: 15 pm
02: 30 pm
02: 45 pm
03: 00 pm
...and so on

User will not be able select following

02: 12 pm
02: 49 pm
... and so on

[wojtekmaj]

Duplicate of #55