i have some possible solutions!
1. if you go to the activity log and find new leaf, it'll tell you when the last time you played was. then input today's date and then the date of the last time you played into this calculator:
https://www.timeanddate.com/date/duration.html (today's date as the end date and the last time you played as the start)
calculate the duration and find out how many days it's been.
then open up new leaf and when you get to isabelle's screen asking "well then, shall we get started?" click "i need help first..." "set the time" take the time that it says it is and input it into this other calculator as the start date
Add or subtract any number of days to/from a start date.
www.timeanddate.com
and then click the "subtract" option from the drop down menu and under "days" input how many days it's been (the number you got from the previous calculator),
then it'll give you a date. set the clock in the game to that new date (and you can even minus a week/month if you want to be more safe).
2. if your activity log doesn't work or you're not sure if the last time you opened the game is actually the last time you played (for example: to write this tutorial, i opened new leaf and now it says the last time i played was TODAY and not actually the last time i played. if i used today's date obviously that wouldn't work) OR you just want to be safe, go back to jan 1st, 2017, the last time you remember playing, or even 2016.
3. i have amiibos of all my villagers! if someone does move out, i can always use the amiibos to move them back in. this one involves buying stuff so i don't like recommending it but it is a possible solution.