How to run a timer every 12 hours (exceeding the max size of int) without an RTC? I'm experimenting with a Nano, and trying to trigger an action around twice a day. I'm aware that it's not super accurate, but I don't really mind, as long as it's in the ballpark.

I've tried using the SimpleTimer library but I'm running into an issue because it takes milliseconds as an int, and 12 hours is 43200000 (larger than 32,767 max). Is there another library, or another technique I can use for this?
Set the timer for every hour, which is within the limit. In your action, keep a static int, initialized to zero, which you increment each time. When it ==12, do the thing you want to do and reset your int back to zero.