PIP form return deadline date enquiry

regarding returning the filled PIP form, they say you should return the form by say 1 December.
so does 1 December also counts? or should the form arrive there before 1 December?

I sent mine with special delivery 2 days before the deadline however postoffice staff said it will arrive on the deadline date as their last collection was already done.

The paperwork needs to be with DWP by the close of business on the date given. We usually recommend giving a bit of extra time in case of delays in the post, but guaranteed next day delivery does what it says!