How to Create a UTC-12 Clock
Occasionally, a need arises to monitor time and date in UTC-12 time zone, also known as International Date Line West. This is the last place on Earth where a particular date "expires" globally. Therefore, it is often used for international deadlines.
Geographically, this time zone scatters from South Pole to North Pole in the middle of the Pacific Ocean, between Australia and Hawaii, with gaps and irregularities, specific to nearby islands and territories. It covers, for the most part, a slice of the ocean, and applies to ships traveling through it, which are the last to use any particular day on Earth. However, Howland Island, USA is located in this zone, so we can use it as our location for UTC-12.
Using International Date Line West Template
If you only need a couple of clocks, one with local time, another with UTC-12, then the easiest is to use the International Date Line West clock template from the
Miscellaneous collection. Import it as described
here and adjust if necessary. For example, you may wish to change a
date format, or a
background.

Using International Date Line West clock template
Adding a UTC-12 Clock Manually
Another option is to add a new UTC-12 clock manually via
menu or
Preferences. The location for this timezone is
Howland Island, USA. Below are the steps for adding International Date Line West clock using the right-click menu.
Right-click on any already existing clock and select the
Add clock option.

Add clock option in World Clock right-click menu
After clicking the
Add clock menu, Preferences opens with
New clock node inserted. At this moment it is set to
Local System Time zone.

New Clock in Preferences is set to Local System Time
We have to change the location to Howland Island, USA. Click the
Change button to the right of the
Location field with
Local System Time in it. The World Map opens. Use the search control on top to filter the list down to a subset of matching locations. Find and select the
Howland Island line and click
Apply button on the bottom.

Selecting UTC-12 clock on the World Map
Change the label for the new clock in Preferences by typing something more relevant into the
Label field, such as International Date Line West, Date Line West, IDLW, or even UTC-12. If you intend to track the date only (not time), you may wish to use
Anywhere on Earth or
AoE label.

Type an appropriate label for International Date Line West clock
Finally, press
OK or
Apply. Below is an example screenshot of how UTC-12 clock may look in taskbar, alongside with two other clocks.

International Date Line West clock in taskbar along with GMT and a local time
World Clock FAQ