Dated covers or stamps on piece allows the plater to reduce the number of probable plates.  If the Stamp is on cover or piece
dated for example with a cancellation of February 4th, it would rule out the possibility of the stamp being from Plate 11, as
stamps from this plate were printed 01 February 1840, and the earliest known date is 04 February.

This list compiled by P.C. Litchfield is a help in determining a stamps plate based upon earliest known dates of each plate:
 
 

Of course, the earlier the date the easier the task.  In the first example below we have a cover from which we can observe the following.  This is an example of Plate 2 on cover.  The c.d.s. on the back is dated AU 3 1840. It is also apparent in this example that the c.d.s. will not always be conveniently clear.  Sometimes the date will simply not be legible.  In this case it is a good idea to see if the contents of the folded letter contains a date.  In this case, if the c.d.s. had proven to be to smudged to read the date recorded at the top of the letter would have been a useful substitution.  The date on the c.d.s. allows us to eliminate Plates 7, 8, 9, 10 and 11.

We also notice the stamp is canceled with a red Maltese Cross.  This helps as it will most likely rule out the later plates which more often bear the black Maltese Cross: Plates 9, 10 and  11.  This allows the plater to simply focus on the remaining Plates 1 through 6.   The final steps towards determining the plate are now the measurement of the check letters using the ruler and data supplied in the Plating of the Penny 1840-1864, Volume 1, and a close examination of the star corners which are also illustrated in the same reference.
 

Sometimes the date is not as helpful.  In the next example we have a date which goes well beyond the earliest known dates of all 11 plates.   What it does tell us though is that it is logically  a later plate.  First we note that the Maltese Cross is black.  Also the stamp itself has a slight gray black appearance characteristic of the later plates.  Also, although I do not show an enlargement of it, this stamp has the "O" Flaw Stage 1.  It is therefore within the group Plates 7 through 10.  The date is not a major component in determining the plate but certainly helps support or reinforce the conclusion of the plater.  In this case the measurement of the
letters narrows it down to Plate 8.  This is the only plate where the check letter T is high to the left and the check letter H is high
to the right.

It was the normal procedure to apply the circular date stamp (c.d.s.) on the back of the letter upon arrival at the destination.  In some cases however, the c.d.s. was placed on the front of the letter. The example below is an example on piece showing the
c.d.s. was placed on the front of the letter See also a detail of the c.d.s. shown  in an upright possession for convenience of legibility.  This Aylesbury c.d.s. is dated JU 4 1841.