Creating a blank "date" field

I guess today is my day for strange questions...

Can I create a series of "date" fields that either start out blank (no date) or that are set to a specific date that is not today's date?

The fields would be used for the "payment date" in the payments area of a membership editor and would cover multiple years.

I could manually set the dates to some very early value manually, but the client would rather not have data in the field unless it was valid data.

Thanks,

Jerry