I agree that the date of the map can usually be found on the map itself.
For the apportionment document it is usually somewhere in the preamble, before the actual plot listings, but is usually written out in words “the year one thousand eight hundred…” so it doesn’t stand out.
Also, if it is important to have exact date information, be aware that the date of the map and of the apportionment document can be different. And that some apportionments have supplemental information from later dates.
I have a copy of Kain & Oliver’s “The Tithe Maps of England and Wales” so if you wish I can have a look and see what they say – just tell me the name of the parish or township, plus the county.