how do I create an internal clock?

Is it possible to create an internal clock that will keep track of time even if the game is not being played?

Animal crossing for GameCube does this, and even games as primitive as Pokemon gold/silver kept time although I never found out how it worked. Can this be done without plugins?

Yes, you need to save the time elapsed in local storage when game exits,

and load it on the next launch and continue from there

you can try rex's timeaway plugin

Without plugins: https://www.scirra.com/tutorials/940/ho ... -a-project

Advanced: Convert each part of the date/time to an integer (total time in seconds). You can then store or compare this value upon startup or save.

Dependent on system time, so can be cheated by adjusting date/time on system.

There may be a way to get the current date/time from a time server, but I am not familiar with the method.