This article is from the Stardates in Star Trek FAQ, by Andrew Main zefram@dcs.warwick.ac.uk with numerous contributions by others.
Objection: in the seventh season ST:TNG episode "The Pegasus", stardate
47457.1, Riker stated that he had had his beard for four years. If 1000 units
is one year then he'd had it for at least five years.
During season one, he was clean shaven. The beard first appeared in season
two (earliest known stardate 42073.1), but was not fully grown. (It had to be
kept slightly short for the entire season for continuity purposes.) From the
beginning of season three, stardate 43125.8, the beard has been fully grown.
It is possible that Riker was referring to the fully grown beard, which he had
had less than five years.
 
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