Well, for those who are wondering: “Past missions? You mean only CI1/CI2 waves/missions?”, NO, this isn’t the type you’re wondering about. The past missions will have graphics look the same as CIDOS(inaccessible until time machine is upgraded)/CI1/CI2 (depending on how many year/months/days you’ve traveled to, but you can’t travel more than 20 years, sorry, no dinosaurs!), and you’ll be able to access them by a time-traveling machine found in the shop (the amount of 
s to get it is 634). They have most of the waves in the present Chicken Invaders, yet they also have some of the waves in the past Chicken Invaders.
Oh, about using the time machine: there will be an option under the “i” flag (it appears once you’ve bought the time machine, and the option’s icon is a clock). If you click on it, it will ask how many years/months/days you will go to (1999 is maximum, and if the time machine is upgraded, the maximum will now be 1997).
Wait a minute, i almost forgot! Once you’ve completed any of the past missions, you’ll have to go back to the present time: 2019 (i wonder if the game takes place in 2019). Anything beyond that will be marked as invalid.