[Margaret Pilkington]

Perhaps that person is going to be out of the country on the 27th of March.
A couple of my friends leave for a three month visit to their son in Australia, they sought advice about what to do with their census form, and were told to fill it in and post it before they go.
Also, you can fill in your census online right now. If they didn't want you to do it then they wouldn't activate the website to do this until the 27th of March.

If you already know the information is going to be correct on that date, I don't see what the problem is in filling it in early.

The vast majority of us live lives of habit and not much will change between now and then........even if we win the lottery!