Not sure how you’re envisioning this calendar, but you could work with a formula field to return the date however you want to use it. You mention the irrelevant time but it’s relatively irrelevant, depending on what you want to see and do.
If what you want is to be able to see a grid layout that says “deadline date” without a time and “event date” with time, you just hide the formula that puts them together. In the calendar, you will see the deadline on top with 12 am and the events in the bottom with their corresponding times–and if you make it so that deadlines have something to distinguish them visually, you may not care so much about the time showing. It’s all about what you’re willing to compromise to use Airtable (and all it’s good things) for free.
For example, would this work for you? (formula)

PRO doesn’t make it much more different but does hide the 12 because it doesn’t exist (2 date fields):
or without the label: 
if you’re not looking for the benefits of the time not being there for deadlines when expanding records on calendar view, and if formatting such as I’ve shown first works for you, you can totally make it work with a formula and the free plan.
Not sure how you’re envisioning this calendar, but you could work with a formula field to return the date however you want to use it. You mention the irrelevant time but it’s relatively irrelevant, depending on what you want to see and do.
If what you want is to be able to see a grid layout that says “deadline date” without a time and “event date” with time, you just hide the formula that puts them together. In the calendar, you will see the deadline on top with 12 am and the events in the bottom with their corresponding times–and if you make it so that deadlines have something to distinguish them visually, you may not care so much about the time showing. It’s all about what you’re willing to compromise to use Airtable (and all it’s good things) for free.
For example, would this work for you? (formula)

PRO doesn’t make it much more different but does hide the 12 because it doesn’t exist (2 date fields):
or without the label: 
if you’re not looking for the benefits of the time not being there for deadlines when expanding records on calendar view, and if formatting such as I’ve shown first works for you, you can totally make it work with a formula and the free plan.
Thanks so much for the suggestions and the time you spent thinking about my
request! That was very nice, and what you say makes a lot of sense –
especially the point about the irrelevant time being relatively irrelevant
The only other problem with this approach is when I sync the data to Google
Calendar (this is the “One Ring to Rule Them All” view of deadline, events,
meetings, other calendars’, etc). My deadlines from Airtable come across as
one-hour events at midnight, so they are basically hidden from view during
the day. I could fix this by separating events and deadlines in the base
and not worrying about the redundancy of the Airtable calendar.
In the end, you are right and I’m not complaining – I really enjoy
Airtable’s flexibility and options. I’m a little obsessive about getting
things just right, but I can do that later when I’m finishing testing and
on the paid plan.
JB