Rebate conditions with mulitple validity dates

Hello,

I have an urgent requirement. My customer wants to define different rebate rate by date in one agreement like normal pricing condition types.

for an example,

The validity date of the agreement is

1/1/2008 ~ 3/31/2008

Rebate conditons are as follows,

Material A $2 per 1unit, 1/1/08 ~2/28/08

Material A $3 per 1unit, 3/1/08 ~ 3/31/08

I would appreciate with your help. I will give you high point!