Might be an initial pain to setup, but you can do a calculated field on a task form like this:
IF(Planned Completion Date < DATE("01/08/2017"), "1", IF(Planned Completion Date < DATE("01/15/2017"), "2", IF(Planned Completion Date < DATE("01/22/2017"), "3", IF(Planned Completion Date < DATE("01/29/...