Pause for an amount of time.
Extremely flexible, with the ability to provide a specific date/time, number of hours, minutes and seconds, etc.
For a complex example of use, check out alarm.sh
Seconds elapsed ∞
To learn the number of seconds elapsed, probably:
\ps --user $( \whoami ) -o etimes,args |\ \grep sleep |\ \head --lines=1 |\ \sed --expression='s/^[[:space:]]*//' |\ \cut --fields=1 --delimiter=' '
NOTE - This might pick up another command with the name 'sleep'. Exactness would require knowing the process number of sleep (which is doable), then checking that specific process number.
Determining the number of seconds remaining is a much more complex problem.