----A---- ----B---- --C--- --D--- --E--- --F--- --G---
  1      Out       In     Jan 09 Feb 09 Mar 09 Apr 09 May 09
  2   22 Jan 09 18 Mar 09     10     28     18      0      0
  3   18 Feb 09 13 Apr 09      0     11     31     13      0
  4   19 Feb 09 04 Apr 09      0     10     31      4      0
  5   03 Jan 09 25 Jan 09     23      0      0      0      0
  6   20 Feb 09 16 Mar 09      0      9     16      0      0
The formula in C2 and copied across and down is

=MAX(0, MIN(EOMONTH(C$1, 0), $B2) - MAX(C$1, $A2) + 1)

EOMONTH requires the Analysis ToolPak.