You can actually ask your Mac to remind you every hour, every half an hour or every 15 minutes by using clock feature, announcing time.
First, go to System Preferences. Click on Date & Time to go inside Date & Time Preferences Pane. Inside here, you will see three tabs, which is labelled from left to right Date & Time, Time Zone, and Clock; Click on Clock.
As you see, this article title is focusing on scheduling, that's mean both of the tasks, i.e. Start Up and Shutdown, are done automatically by your Mac. What a save of energy.
I'm not forcing you to set up according to my opinion. I just want to let you know how this work on your Mac and what is the advantage and disadvantage.
This feature gives you advantages: time saving as you don't have to wait for start up time and energy saving as your Mac will shut down automatically even if you've gone to the dream world. Of course there is disadvantage: sometimes annoying: as it doesn't care whether you're using the computer or not, i.e. when you're in a rush for working deadline, suddenly your Mac automatically shutdown, what a nightmare..
Frankly say, that's just my opinion, if you've another point of view, feel free to reach me.
To set up your date and time, first you have to go to System Preference and under System grouping, look for Date & Time. You also can use your QuickSilver for quicker access by using keyword
Mouse can do for your this favor too, you can take your mouse to click on Date & Time icon.
Now you've already been inside Date & Time Preference Pane.