Is "...つもりだったでしょうか" an OK translation for "Did you mean to ...?" A person planning a seminar wrote the date and day of the week, but the day of the week was incorrect. So I wanted to send a message asking "Did you mean to write ...?", but I'm struggling to wrangle a natural-sounding translation. The closest I can come up with is ()but for some reason something about it doesn't sit right with me. Could someone with a better "natural Japanese" filter give a better idea?

I suppose _do you mean to_ would require some modifications depending on what you are asking.

Speaking backward, / normally sounds like _do you plan to/did you plan to...?_. That is, it sounds asking about the explicit plan, not implicit intentions. So if a sentence can be rewritten as _do you plan/try to...?_ , probably can be used.

As for the particular example, suppose (this year) there is written 319actually we normally say

1.

2. 19

3. 1920




In 1. you generally ask about the inconsistency; in 2, you assume the day of the week is wrong; in 3, you assume the date is wrong.

Slightly more generally, I guess you need to rephrase the English to something like "Currently X (which puzzles me), did you mean to write Y?", and translate it as "XY" (with possibly many modifications depending on the context).