The last scene is for you. They actually go back in 16th may, the day of murder.

The system of the car just bring them go to same date and month but in different year.

So when they in 1987 after they catched the murderer, the time already on june 1987. So if they want to go back they just can go to june 1986 cause they can't setting for the date or the month, they just can set for the year. Ofc they went to 2021, their current time. If you notice, FL disappear for one month in 2021.

Thats why in the last scene FL asking him to go back cause now they on 15th may 2022, and tomorrow is 16th may the day of 1st murder (FL's aunt and the teacher death day) . We can assume they want to save the  others victims.