Column "A" Is My Date column Which Starts with Jan 1, 2000
Column "B" Has Generic Data

Now In column "C" row number corisponding to Jan 1, 2001 (Cell "C55")
I have this formula , =IF(ISNUMBER(B55),B55-B54,"")

Now the problem I am having is that when I do a fill down of this
formula Cell "C56" Formula looks like this =IF(ISNUMBER(B56),B56-B55,"")
What I want Cell "C56" Formula to look like is this,
=IF(ISNUMBER(B56),B56-B54,"")
and the next cel should be
=IF(ISNUMBER(B57),B57-B54,"")
ect... ect...and so on.

How can I do this using some sort of fill down so that I don't have to manual
change each formula for every cell, as I have about 5000 cells and that
would take forever.

Thanks in advance for you help
Dan Thompson