TimerPlugin
Spigot plugin for timers, useful for parkour. Tested on 1.17.
Commands
/starttimer [player]: Starts and displays the timer for given player.
/stoptimer [player]: Stops the timer for given player.
These commands are preferrably to be used by command blocks. I recommend using this plugin for allowing selectors to be used with plugin commands:
https://www.spigotmc.org/resources/commandhook.61415/
Permissions
timerplugin.starttimer
timerplugin.stoptimer