From DGT GTD
(long click on "Due date" to enable/disable floating date, a green dot will appear on beside the due date)
A typical event is suitable for something that has to be done on a fixed day and eventually at regular intervals. But suppose you have something that needs to be done but not necessarily on a specific day. For example, you may have to give your dog a worm pill once a month. It’s not critical that it be given on a specific day and in fact if you don’t get around to doing it on that scheduled day, you really want a reminder that will remind you each day until it gets done. This is exactly what floating events are designed for. You set up a floating event and when that day comes around, DGT moves the floating event forwards until you check it off.
Warning for Toodledo users!
DGT GTD floating feature is mapped to the "Optionally on" in Toodledo: https://www.toodledo.com/info/help.php?search=optionally
Quoting: "if the date passes and you have not completed the task, the task will be automatically removed, or moved forward if it is a repeating task."
Then if you need/want to keep the DGT GTD behaviour of the floating date, you have to set a repeating option to "Daily" otherwise Toodledo will remove the task.