If a target date is set, you can use the special strings ${target_days} or ${target_years} in the event text to display the number of days or years since (or until) that date. You can also use ${target_date} to display the date itself.
${target_days}
${target_years}
${target_date}
For example, if
This works with repeating events too, so the displayed interval would change based on the date of the repeating instance.