@Alec_Krohto you couldn't do this with Butler, but this Trellinator function would do it:
You can read more about Trellinator here:
Huh, a bit late, didn't think this was possible but I think I found a way with a Card Button.
Just to be sure, you just want a way to fill in empty date custom fields after the fact right? The important bit is that you can filter card collection (to a Checklist) by whether a customfield has been completed or not (filled in), and then act on the checklist.
You'd have to hardcode the input date you want every time.. It might be possible to set the startDate on the trigger card, to be copied in to all the found cards, accessed somehow via Variables, but that escapes me for now.
Edit: in hindsight, not having tested this fully, the last two lines may or may not short-circuit the loop.. If it does, you can get rid of it and just remove the toUpdate checklist manually.
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