I know that tomorrow I will have a higher credence in it being Monday than I do today, and yet it would not be rational for me to increase my credence in it being Monday now on this basis.

It seems like if you cash out the concept of "it being Monday" in terms of timeless facts about the block universe, that there is no change in belief only time. Your strength in belief in the referent of the quotation "it is Monday" changes only because the referent itself changes.