If I PD a reserve day in May and subsequently earn a banked PB day in April, can I retroactively put that PB day on top of the PD, be paid for the day, and GS on top of it? If not can I just GS on top of the PD?

EDIT: LOL, not that it matters. I just looked and there aren't any blue days in May anyway. What was I thinking? Anyway, if somebody knows it would still be nice to know for future reference.