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This must be a simple question for a native speaker.

I know that we use "on" with dates: I'll see you on January 1st.

And we use "at" with times: I'll see you at 17:30.

But what preposition has to be used when we speak for date and time:

I'll see you on January 1st at 17:30. looks ok.

But what in this case: It happened on 2014-01-01 17:30.

Is "on" correct when we are specifying the date and the time?

The date-time comes as a ready text as 2014-01-01 17:30 and I cannot modify it. I can only put text before the date-time string or after it.

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  • Few native speakers would ever write the date and time like your 4th example. It would almost always be written like your 3rd example (or some variation thereof)
    – ElendilTheTall
    Jul 7, 2014 at 11:41
  • I need this text in log written from software product. There is an event and a time stamp. I have to put some preposition between them.
    – Miroslav Popov
    Jul 7, 2014 at 11:42
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    In that case it would be exactly the same as your third example - the construction is the same - on DATE at TIME
    – ElendilTheTall
    Jul 7, 2014 at 11:43
  • 4
    Time of occurrence: 2014-01-01 17:30
    – mplungjan
    Jul 7, 2014 at 11:53
  • 1
    If you really are limited to one preposition I would use 'at', since the date & time as stated is precise to the minute. That requires 'at'.
    – WS2
    Jul 7, 2014 at 12:24

4 Answers 4

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36

The construction for your fourth example would be the same as your third:

on DATE at TIME

While most people would understand it without the AT, it is strictly correct to include it.

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  • 5
    I finally chose a single preposition form with "at". Because the time is the meaningful info. A real example from the software is: Big Gap of bar 403 at 2007-04-01 20:00.
    – Miroslav Popov
    Jul 7, 2014 at 19:55
  • @ElendilTheTall, So is it at 8am 2016-06-06 or is it on 8am 2016-06-06?
    – Pacerier
    Nov 18, 2016 at 2:16
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As far as I know from my teaching experiences; you should consider the smallest time unit when using a prep. As follows; It happened in 2014 It happened on 2014-01-01 It happened at 2014-01-01 17:30 I hope this helps.

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  • This does make sense, as this defines precision of date-time value - speaking about year, month, day, or exact time.
    – kravemir
    Nov 8, 2020 at 11:05
14

In computing, the notion of date and time is often amalgamated into a single entity, a datetime.

It may be required that a datetime is presented in a particular format. An example datetime expressed in ISO 8601 may take the form 2019-06-12T14:07:38Z. The problem is that these standardized datetimes do not naturally blend into English text. Neither It happened on 2019-06-12T14:07:38Z nor It happened at 2019-06-12T14:07:38Z are standard English.

I would argue that 'at' is the better option since we are referring to a moment in time rather than to a day as 'on' would imply.

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  • 1
    While this is useful information, I don't believe it answers the question from 2014, which was about the difference between using at and on when using plain language dates and times.
    – Davo
    Jun 12, 2019 at 14:45
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    @Davo Please read the question author's comments. He is coming from a computing background and is asking precisely about the scenario in my answer.
    – user33313
    Jun 12, 2019 at 15:05
  • Like this one: I have to put some preposition between them ? It's not a question about formatting date time stamps, it's a question about the use of on and/or in when listing dates and times in plain language.
    – Davo
    Jun 12, 2019 at 20:12
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    @Davo 'I cannot put "at" between the date and time. The time stamp comes as string like: 2007-04-01 22:00' - miroslav-popov
    – user33313
    Jun 13, 2019 at 7:16
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    @GrantZvolský is correct. The time stamp comes as a ready-made text. It would be overkill to parse it, having into account that the exact format is unknown, only to set a preposition.
    – Miroslav Popov
    Jun 13, 2019 at 9:21
7

You would always use both.

Let's do something at TIME on DATE

You could abbreviate the "at" out in some circumstances but only when facilitated by grammar and cadence... And even then, it's still implied:

Shall we meet on DATE... around TIME?

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