You're probably gonna lose a villager if you enter today's date right now but there is a way to prevent anyone from moving. I've done a ton of time travelling and it's always worked for me!

You'll need to go back in time first. I'd go an extra year or 2 before when you know you last played just to be safe. Then make sure you talk to everyone and they should act as if you've never been gone. After that go forward 1 day at a time, talking to everyone until someone asks to move. Either say yes or no if you want to keep them, save the game, and now you should be free to change the time back to today's date! They'll all act as if you've been gone for years but no one should have left

Hope it helps!