custom date validation format for dff field

[yo_jyo]

requirement -
dff has a specific date time format different from the standard valuesets for date-time stamp available in apps. So if the user enters the incorrect format, it should do the validation.

Format needed for dff is : 2012-04-30 15:46:35

I tried a couple of thigs but dint fork . How do I do it.

any suggestions ?
Thnks