Time

The panel of the Time preferences is split into four tabs:

Date and time

On the left side, you can set the day of the month by simply clicking on it in the calendar. You change the month and year by clicking on it and using the up/down arrows to the right or the cursor keys on your keyboard.

Similarly, you set the time at the right. Or simply move the hands of the clock directly.

Time zone

Simply find and choose your country in the list of continents and press Set time zone. For countries with more than one time zone you'll have to expand one level deeper.
To the right you'll find the time of the currently set time zone and the preview time of the the zone you've chosen.

Also on the right side is a setting for the hardware clock of your computer. There are two modes for the clock:

Network time

Setting date and time manually is pretty much obsolete if you're connected to the internet. There are public servers that supply very exact time signals.

With the +/- buttons you can add/remove NTP-servers to/from the list.

The checkboxes below tell the system to Try all servers when synchronizing, not only the currently selected one, and to Synchronize at boot, so you can be sure you're clock always shows the correct time.

You can Reset to default server list if you accidentally deleted working NTP-servers and Synchronize your clock manually. The synchronization can be achieved from the command line (or a script) as well:

Time --update

Clock

The last tab shows the options of the clock displayed in the Deskbar. You can disable the clock there completely with the top checkbox. The rest are equally self-explanatory.

In every tab is a button Revert that brings back the settings that were active when you started the Time preferences.