[Madd92]

If you want something to happen (an action performed by the server) after 5 hours I don't think using only timestamps will do, you will need a timer for that. But if you are using a command to show the status of the plant for example, you can just use the timestamp to determine whether 5 hours have passed since planting.
So if you just want to check the status when somebody uses the /harvest command for example you could just calculate the difference between the saved timestamp when the weed was planted and the timestamp when the command is executed.