It appears that the CDU has a nifty little feature buried in its GPS time settings. If you have a set of waypoints you need to hit at a certain time, DTOT is an excellent feauture that gives you desired airspeed and early/late time of arrival on the HUD.
Normally you'd need to know the push time to set the WP's DTOT, and do the math adding the push to the relative time.
You can actually punch in all the relative times in the WPs on the ground. Once the push time is known, go to SYS > GPS > TIME, type the time in HHMMSS format in the scratchpad, and hit LSK1 (DTOT OFFSET). All the WPs in the DB with a DTOT set will be updated with the inserted time added.