This is a pretty simple mod. It shows the In-Game Time on top left of the screen.

It can be configured to

Show-hide the 'In-Game Time: ' string
Set the Update rate of the timer (by default each tick)
Press 'o' to stop the timer

Config File:

#Show/hide the 'In-Game Time: ' string

show_text = true

#Each how many ticks should the timer be updated? (20 ticks = 1 second)

update_rate = 1

Old 1.12 features and config

Show-hide the timer
Show-hide the tick count
Don't start the timer at the world load, instead use a command '/igt start' to start it and begin able to stop it with '/igt stop'.
Show the In-real-life time (with configurable format, hh:mm AM/PM by default)

Configuration File:

# Configuration file

config {

    general {
        # Show/Hide the 'In-Game Timer:' text
        B:"Show Text"=true

        # Makes ticks appear after the time, in parenthesis
        B:"Show Ticks"=false

        # Show/Hide HH:MM:SS:DD time
        B:"Show Time"=true

        # If enabled the timer will start when the command 'igt:start' is executed instead at the start of the world
        B:"Start Timer via Command"=true

        # Every how many ticks (1 second = 20 ticks) should the in-game-timer update?
        # Min: 1
        # Max: 20
        I:"Update rate"=1
    }

    "real time" {
        # How to display the irl time. Possible pattern letters can be found here: https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html
        S:"Real Time Format"=hh:mm a

        # Makes the string on top left corner show real life time. If this is true, the in-game-timer will no longer be visible. In 'format' you can even add day, month and year.
        B:"Show Real Time"=false
    }

}