This is a variable number. You need to state a start and/or end date or time.
21 of them.
Between March 11 and May 21, 1963 there were 10 weeks and 1 day.
21 weeks and 5 days!
21 weeks and 6 days
4 weeks pregnant
21 weeks and 6 days.
As today is January 27, 2010 there are 21 weeks until 23 June, 2010.
As today is Janaury 7 there are 73 days to go until March 21.
The 31st of March.
between 20 and 21
From December 28, 2009 until March 21, 2010 there are 83 days inclusive.
North pole have no sunshine at all from September 21 until March 21. South pole have no sunshine at all from March 21 until September 21.