I hate to be picky, but because of the way you've phrased this, I'm not sure what you want. You said you needed a query to return the most recent date entered into the subform date field. But a query retrieves data from a table, not a form. So I'm trying to figure out how the form comes into it.
Do you want the latest date in the subtable, period? That's easy: Create a query on that table, drop the date field into the grid, make it a Totals query (choose View>Totals), and set the Totals line for the date to Max.
Or do you want the latest date entered via that form, as opposed to those entered via another form, or via a table or query datasheet? To do that, you'll have to store another column in the table, to indicate where the row data comes from, then use that column in your query criteria. (Other than that, use Max in a totals query as above.)
Or do you want the latest date entered into that form in the current session, that is, since the form was last opened. Or something else?
If I haven't yet guessed how the form fits in with your question, please provide additional info. Rick Sprague