Such calculations typically include a reference to Get ( CurrentDate ). Calculations that refer to this function must be Unstored in order to update correctly with the passage of time.
Double click the field definition in Manage | Database | Fields to bring up the Specify calculation dialog, then click the storage options button in order to specify that this field be unstored.
Hurray Hurray Hurray!!! What a simple solution - but one I was never aware of! Thanks so much!!