I'm thinking of gifting a one year post dated cheque as a wedding present
Why would you want to give a check that can't be used for a year?
I think it would be funny to give them something that they can't use yet.
Some people do not really want money for gifts, especially if it cannot be used right away. You should consider getting them something that they can benefit from right now.
Why can't they just use it after a year?
They might need the money now, and it's a little inconsiderate to give them something they can't use right away. What if they lost the check or forget about it? How about just asking what they want so they would appreciate it better.