I find it is easier to "fall through" to get a result. The formula is simpler.
You might try this:
Code:
IIf(Date()>=[Platinum],"Platinum",IIf(Date()>=[Gold],"Gold",IIf(Date()>=[Silver],"Silver",IIf(Date()>=[Bronze],"Bronze","Stanadard"))))
Given these dates,
[Platinum] = 6/30/2013
[Gold] = 5/31/2013
[Silver] = 4/30/2013
[Bronze] = 3/30/2013
If Date() = 7/1/2013, the result will be "Platinum" (Date() > [Platinum])
If Date() = 6/15/2013, the result will be "Gold" (Date() >= [Gold] and < [Platinum])
If Date() = 5/15/2013, the result will be "Silver" (Date() >= [Silver] and < [Gold])
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My $0.02