Formula Question

Okay, this may be a difficult one. I have a simple table layout like this.

Company (4,0)
Year(4,0)
Period(2,0)
Amount(15,2)

I want to be able to take an ending period and determine 12 months back from
the ending date and select records in the DB via a select statement. We are
really close with the following formula: ( ( ( (period-12) / (period-12) ) *
12 ) - 11 )+period. A co-worker of mine got this far. If it wasn't for the
whole divide by zero bit as this portion ( (period-12)/(period-12) ) would
for period 12, it would work great. Short of a case statement is there a
better way to handle divide by zero? Or is the a better formula that we
aren't seeing?

--
Mike Wills
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http://mikewills.name
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"There are 10 types of people in the world: those who understand binary, and
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