I tried that first. Masterdata column H is and has been in a number format which is why I'm very confused. Now, Masterdata column G [Year] is not because I'm not sure how to format just a year as a date column. However, the years are sorted fine so I'm not sure this matters. Is it possible excel doesn't see consecutive dates and therefore doesn't want to group them by month? Somedays there is no overtime taken hence gaps in the date sequences.

I'm so close but still a step away.