Code cleanup for Clock & Calendar

[mentusfentus]

What's changed?

This PR cleans up some of the code of #2397. This also:

• Fixes minor oversights;
• Changes the name of variables to be a bit more descriptive.

Where have you tested it?

Nintendo DSi XL with Unlaunch melonDS 0.9.5

Pull Request status

• [X] This PR has been tested using the latest version of devkitARM and libnds.