Time Picker : Custom Values

Hi,

I would like to have a time picker, which the user can only specify the minutes as either 00, 15, 30 or 45.

Can this be done. I have noticed with the dateTime picker that Custom Data can be defined, but I have no idea how this would be done.

Someone else has asked the same question , but had no response.

Any help is much appreciated.

Thanks

Martin