Time notation, is a dot valid between hour and minute?

This is the first time in my life I’ve ever seen the xx.xx notation. If I saw it in context with an am or pm, I’d understand what it meant but I would assume it was a typo. If it didn’t have the am/pm, I’d likely have no clue that it was meant to be a time.

In the U.S. we never use anything other than :

Definitely a British/ Australian thing.

I use a dot all the time, especially when writing on a phone, where the period is much easier to access than the colon. US.

I could be wrong, but I believe this is a holdover from older systems like telegraphs (or texts on old phones), where sending a period was easier than trying to signal advanced punctuation. In certain note formats, this is easier and quicker to write than a full colon without obscuring the meaning. In the US it wouldn't be common in things meant for public consumption,but a memo like this references probably wasn't meant for outside eyes.

In the US: Valid? Yes. Common? No. It is more commonly used for dates than times, as in 11.26 would mean November 26th, but it's not very common for that either, as we usually use slashes (11/26). Either way, using a period would cause the reader to do a double take, but would be understood.

It's not what we use. A good chunk of people will still get it, but another good chunk will stare at it and completely fail to grasp what you're saying.

There are some forms that I fill out that require I put in a colon. Other than an instacnce like that, I always use a period between the hour and minute. Everybody seems to understand. I'm an American.

You often see the period used for times in 24 hour format. Not sure why it's here while also being followed by "a.m.", perhaps the original source used 24 hour time but the writer of this text decided to include the "a.m." for ease of readership?

The answer is... It depends on the style guide of the publisher. In the UK, the BBC style guide requires a colon, The Guardian newspaper requires a full-stop. At school (in England) I would have been taught to use a full-stop for 12-hour notation and no separator for 24-hour notation. So both are valid, just go with something that has consistency

I'm English, and yep this is absolutely normal for me - I'd generally write 11.42am

So the original British form was for a dot. You've not told us where this is from but if it's a British book some decades old that might explain it. (I realise that's unlikely with the 2001 but who knows.) These days we use the American style colon between hours, minutes and seconds. There is clarity there in case you have to do fractions of a second for sure.

A single dot is ambiguous because it could indicate a decimal, which is 15 and 42/100 hours not 15 hours and 42 minutes. Sometimes computer systems do deal in decimalised hours instead of hours and minutes, so I’d avoid this notation.

Yes it is absolutely valid and correct.

11.42 means "eleven and forty-two one-hundredths. 11:42 in the right context means it is forty-two minutes past eleven. Just because typing a colon on a phone requires one extra button push than a period doesn't mean it's advisable to try to change the way an entire culture expresses time in writing. I don't mean for the tone of this comment to sound harsh, but I see what I think is too much change for change's sake. The colon works. Why mess with it?

between 11 and 42? yes, 11.42 or 11:42 would both be common ways to write that.

At most, I believe I've occasionally seen time expressed as 8.5 AM, which is meant to convey 8:30. And that would only be because you'll more often see a decimal point to convey a period of time. A meeting might be 2.5 hours long, for instance. I would say that this notation would confuse me very much, beyond a brief initial impulse to try and calculate .42 hours. But it's definitely nonstandard in American English.

1142am looks strange to me, with or without a gap, probably because 1142 looks more like a year than a time. So yes, . or : are necessary. And by the way, in British English, am and pm don't need dots, because context makes it clear that it's a time and not "I am ... whatever." Using capitals is also weird, because the abbreviations are not proper nouns, so A.M. and P.M. are really overkill. And Microsoft, it hurts my eyes to see 07.30 A.M. or 21.15 P.M.

It looks weird but legible to me. I'd assume it's a non-American way of writing time if I saw it in the wild.

I don’t know, but I believe the best option is writing it like a regular acronym. (AM, PM, etc.)

Perfectly normal usage to me along with : 12.42 12:42

other countries are weird

I don’t know about other English speaking countries but this is not correct in the US

People would figure it out due to context, but it is not valid in the US. Use : here

I see dots sometimes for like bus and train schedules

No

No. Time notation uses a colon : exclusively. Dates will commonly use forward slashes, as in dd/mm/yyyy. For a time following the early advent of the internet it became fashionable to use dot in dates, as in dd.mm.yy. For a VERY brief time, dots also replaced dashes in phone numbers, but this is now no longer the case.

I'm British and only ever see times written with a colon, so 11:42. I think if they used a colon they wouldn't need the am, as it's clear it's a time. Without the colon, the am tells you it's a time.

Native speaker here, Midwest US. I don't frequently do it, but I will use a dot as shorthand now and then (especially when writing by hand - when typing, I always use a colon). Edit: most commonly I write in a 24-hiur format (ie, 0830, 2130, etc).

Not in American English, no.

I understand that if you use am/pm, then you use a colon. If you are using 24hr format, you use a period between the hour and minute. In the US, 24hr time is unusual except for in healthcare and research.