If you were to build a clock and had to choose between indicating AM or PM, such as with a single LED indicator, which would you choose and why?

I have seen commercial clocks using both approaches, with the LED being lit for AM and PM, with PM being more prevalent, but is one better than the other?

  • AM, probably? Most people are awake for more of the PM than the AM, so to have the AM as the special case seems more logical. But what leads you to have to indicate one or the other and not both?
    – Oleksandr R.
    Oct 4, 2015 at 17:28
  • If you have a single LED indicator, using both would only lead to confusion.
    – coding4fun
    Oct 4, 2015 at 17:31
  • If you have a single led indicator, you can indicate both. It's either AM or PM, can't be both at the same time. So either have "on=PM, off=AM", or the other way around.
    – Tom Carpenter
    Oct 4, 2015 at 17:43
  • Well, that is exactly what my question is. Should I have the LED on for AM or PM? Sorry if I did not make it clear.
    – coding4fun
    Oct 4, 2015 at 18:05
  • I don't think there's any EE justification for choosing one or the other. @PaulA.Clayton is right; this is probably a better fit for the UX SE.
    – Adam Haun
    Oct 4, 2015 at 18:36

3 Answers 3


If you were to build a clock and had to choose between indicating AM or PM, such as with a single LED indicator, which would you choose and why?

Which is the ambiguous condition? If the clock reads, say, 9:00 - which situation is ambiguous?

Evening is when it's ambiguous. Is the clock military clock? Since you only have one light, have your LED light for indicating evening.

The reason PM is better is because it helps clarify the more ambiguous situation.

Consider an interface like this:

enter image description here

It can just read nicely.

  • 1
    I like your approach - 9:00 without any special decorations is the same time, which is 9 am, whether a 12-hour or a 24-hour format is used. PM needs some extra disambiguation. This makes it easy to remember what that extra LED stands for, for me at least. However, I am not certain if this logic can be easily explained to consumers if this is a mass produced clock.
    – mcu
    Oct 5, 2015 at 1:43

The approach in question is not a good approach since it includes an element of cognitive friction that will make things really hard for an user. If there's a technical reason to use just one LED, then it would be better to look for a different approach. Either AM or PM should be lit at any time as it fits.

There really isn't much more to say to this, UX is not for "guessing", it's meant to make user's experience better.

An alternative path, assuming this is a digital clock, would be to use a 24 hours clock (also know as military time), but this approach would be useful only in countries where this way of measuring time is commonly accepted.

  • I just think that if you use a two LED approach, one LED for AM and one LED for PM, it can be hard to tell from a distance which LED is on. And of course it could add to the cost of the clock, if that is important. I only wish all manufacturers stuck to the same convention - either the LED is lit for AM or PM. That would make it easier to remember what is stands for.
    – mcu
    Oct 5, 2015 at 1:35
  • 4
    Note that the AM/PM functionality is not used for normal reading of the clock, since a user almost always knows whether it is morning or evening. When this is used is 1) setting the alarm, 2) setting the time on the clock. As such, I think a single LED is adequate. However, it means the "reading from a distance" objection isn't so strong, either.
    – user31143
    Oct 5, 2015 at 6:51
  • Also note that 24 hours clock is more popular in some countries. I.e. in Poland we sometimes use 12 hour system (without AM/PM) in speech, but in writing there is used 24 hours almost exclusively.
    – Hauleth
    Oct 5, 2015 at 14:45

Analog clock faces don't have an A/PM indicator because they're never mistaken for 24h clocks. In digital clocks, I'd try lighting​ only the lower dot of the colon for 12h clocks, because "1.23 pm" is much better established than "13.23". You could then also test only upper dot = PM.

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