Question about tracking usage

I am curious to write a plugin which tracks time a player has been in a world.

1- record when each player connects
2-tell them they played when they send a custom command (ie /timeonline)

I am stuck on getting the time a player connects. I *think* I understand how to do #2.

Can you suggest how to start writing #1?

Initialize an array field, set the key as $player->getId() and value as time() or microtime(true)

Thank you. Works perfectly