Since last evening, a strange error has crept up

Since last evening, a strange error has crept up

My formula calculates days between two date variables.

(Checks_Cashed - Date_Delivered / (1000 * 60 * 60 * 24))

Until yesterday, it was working fine. Now also it works fine. But when I try to create another similar formula, or even save the existing formula, the system reports this error:

Error in statement at line number 0
The template variable Date_Delivered is of type TIMESTAMP. But a Integer Expression was expecte
immrajiv