I have an application in which users can enter tabular data such as:

start               | value
------------------- | -----
12:30:00 27/10/2019 | 0
01:30:00 27/10/2019 | 1
02:30:00 27/10/2019 | 2

It turns out that 27/10/2019 02:00:00 (date given in UK format, so 10/27/2019 for Americans) is the "roll back" daylight savings instant for the UK. That is, at 2am, clocks are rolled back to 1am. That is, all times between 1am and 2am occur twice, and there is effectively a 25 hour day. As such, the second row in the table above is an ambiguous time. Does the user mean the first occurrence of this time, or the second?

What is a good way to provide UX around this (This is an html/js web application so there are quite a few options graphically)? Adding another column to the table seems extremely verbose just for this edge case. Displaying date-times in UTC is not an option either, as this would make the day-to-day usage of the application quite difficult for users.

  • 1
    I am not entirely certain of how daylight savings works, but from what I know (and please correct me if I am wrong), this scenario should only occur once in a year, when the clocks are rolled back, right? Apr 16, 2019 at 7:35
  • That is correct, it only happens once a year. It does, nevertheless, happen, and the application is a real-time production tool on an industrial site which operates 24 hours a day, every day of the year. As pointed out by Glorfindel, the hour is deliberately chosen so that for most applications it is a non-issue. Even Google Calendar does not wrap any UX around this (you simply can't create events for one hour of the year). For this application, however, it is likely that it will be an issue, as operators will certainly be using it at that time in its once-per-year occurrence.
    – Coxy
    Apr 16, 2019 at 22:13

1 Answer 1


These timestamps should be very rare; it's just one hour in the 8760 which happen in a year's time. The hour is deliberately chosen in the night, so most people not familiar with UTC (programmers, system administrators, astronomers, military) don't need to register anything in that hour.

That said, one option might be that when a user enters an ambiguous timestamp like 01:30:00 27/10/2019, they are prompted if they mean the one during Daylight Savings Time or the one outside. Afterwards, mark such a timestamp with something like an asterisk; that takes little space and provides enough indication that there's something strange about that timestamp. When a user clicks on the timestamp (or hovers over it), you can show another popup where they can change their 'DST or not' choice.

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