ElapsedTime(DecimalTime; format) creates a special string which generates a running time on lists, showing the time elapsed since time.

If format is not given, the default format HH:mm:ss will be used.

If the format starts with a ”-” (e.g. "-HH:mm:ss"), it will show a countdown to time, followed by the elapsed time from time.
If the format starts with a ”+” (e.g. "+HH:mm:ss"), it will also show a countdown to time, but the elapsed time after time will be prefixed with a plus sign.
If no sign is given, the output will simply start showing the elapsed time from time, without any countdown or sign prefix.

Note that ElapsedTime will not work for multi-day events where the time exceeds 24 hours.