How to modify fromdate and todate in prices condition via bapi function?

I got into trouble when calling bapi function "BAPI_PRICES_CONDITIONS" to modify the prices condition of purchasing inforecord, it can be used to modify the price successfully, but the fromdate and todate of condition can not be modified. How to modify the fromdate and todate in prices condition via bapi function? Is there anybody can help me? Thanks in advance.