Simple and completely client-sided mod that renders vanilla clock and system time in the game overlay:

Previously was a part of Enigmatic Legacy. I mainly made this for myself, as one of few people who prefer to play in fullscreen mode (because of resolution issues), where you can't easily see the time, which for me is absolutely required to be easily visible.

There is also a config file (UniversalClockHUD.toml) located in the config folder, where you can make following adjustments:

Set the clock to be only visible in fullscreen mode;
Set whether or not the clock is hidden when chat is opened;
Enable or disable background behind the clock;
Choose where the clock is positioned among multiple presets;
Enable 12-hour time format if you're not a fan of standard 24-hour option;
If default presets aren't enough, define custom preset with adjustable anchor point and X/Y position on the screen;
Stop the numbers on the clock from spinning uncontrollably in the Nether or The End (which they do, by default);
Require vanilla clock to be present in player's inventory in order for them to see the HUD;
Make the clock display in-game time instead of system time;
Enable or disable the clock entirely.

By default, the clock is located in left-bottom corner of the screen, but you can freely adjust it to be anywhere on your screen by editing X and Y position in the config file. Notice, however, that if you change those values, they might behave differently with different gui scales and on different screen resolutions. You will have to create personal adjustments that will work perfectly with your resolution and GUI scale.

The default appearance of Universal Clock:

Universal Clock HUD