Correct preposition for dates written YYYY-MM-DD

What is the correct preposition when writing dates in the numerical format YYYY-MM-DD? I would argue that there are two possibilities, "on" or no preposition at all, but then again I'm not a native English speaker.

So which if these would you consider (most) correct:

"The experiemnt was carried out on 2019-05-21."

or

"The experiemnt was carried out 2019-05-21."

?

Generally, events happen in a year, in a month, on a day and at a time. Whether you use the YYYY-MM-DD format recommended by ISO-8601 or some other format should not really have a bearing on the choice of preposition.

Here's a usage example, correct in my opinion, from the Wikipedia article on ISO-8601: 'An interval denoted "2007-11-13/15" can start at any time on 2007-11-13 and end at any time on 2007-11-15' (emphasis mine).

These numbers are pictograms that represent a date, so they are not words and they are not English. You can see this because you could write the date like this in any language. However when you read it you will have to read it in English. Perhaps you might say "on the twenty-first of May, 2019." But of course this will vary by dialect and personal preference. You might even read out the numbers as written, although this would sound a bit strange. But you still have to make a choice based on dialect. In particular, on is usually considered compulsory in British English but it is acceptable to have no preposition in American English.

In short you need to decide what sounds right when you read it in whatever variety of English you are using.