since patch 11.0 was released in Bloons TD 6, treasure chest rewards were changed completely, and we have been missing up-to-date data.
I noticed the rewards repeat after 385 days, anyways I expect the period is shorter. As 385 = 5 × 7 × 11, the cycle is probably 35, 55 or 77 days.
You should be able to count your relative position in the reward cycle, so please, help us to fill in the mission info on the page.
You can find the table on Treasure Chest (BTD5 and BTD6)#Bloons TD 6.