Several things going on here.
First, to address your main question, significan digits is not the problem.
XL can handle 15 sig digits, and your meter only goes up to 8. Second, by
changing the cell format to Text, the logic check of:
Z15Y15
where Z is text and Y is a number, will ALWAYS result in true. While this
may give you the correct result, because XL is nice enough to convert the
text "00050" into the number 50 and then subtract, its not really a good
logic check.

Unfortunately, the real problem here is undefined, since you only wrote
"Problem developed that caused the IF product results to be backwards."
While this may be true, w/o examples of your data, its harder to diagnose
what went wrong. If Y & Z are formatted as numbers, and data is input as
numbers, your formula could be:
=Z15-IF(Z15Y15,Y15-100000000,Y15)

--
Best Regards,

Luke M
"Eiyore" wrote in message
...
I have been using a simple IF formula w/o any problem for several weeks
until
now. The IF statement just calculates the amount of product from a
meter
every week. This simple formula compares the 1st of the week vs the last
of
the week readings and even if this meter rolls over this IF statement
calculates out the correct amount of product.

IF(Z15Y15,100000000-Y15+Z15,Z15-Y15) Y=1st of week reading & Z=Last of
week
reading.

The meter goes up to 99,999,999 units. Problem developed that caused the
IF
product results to be backwards. After noticing this I flipped the to
and it worked correctly to my surprise - but how? After trying Cell
Format
options w/o any luck I tried placing a zero in front of the "Y" number
when
typing it in with Cell Format set on Numbers w/o decimals (sometimes the
meter has just rolled over and the number is small w/one or more zeros in
the
ten million place and downward in value). I now keep the cell in Text
format
and place the zeros in (ie: 00012345) and this works fine as well but why
has this issue just begun presenting itself? There has been many same
type
scenarios in the past w/o any issue. Also the file is always renewed
every
week using the "Save as" option and just renamed.