Sorry but you are not correct. There is no such thing as 00:00 am

https://en.wikipedia.org/wiki/12-hour_clock

Correct there is no such thing, but I never stated that either.
Brian please look at my screen shot.
When you select 3 PM, the input field above shows the (24 hour time) 15:00:00
When you select 12 AM the input field above should show the (24 hour time) as 00:00:00

@GCLW setting the time format parameter is the one part of the equation, the other part is to provide the correct format. This is not done automatically by the field

It is done by the Java Script. When you have either [1-11] selected as the hour in the time picker, and if you flip back and forth AM or PM, in the date/time field above the hour has +12 added to it when PM is selected.
Look at the screen shot.

This might help.

%H - Hour of the day, 24-hour clock, zero-padded (00..23)
%I - Hour of the day, 12-hour clock, zero-padded (01..12)

Again your format is wrong

The "timeformat" attribute dictates how the drop-down menus will be in the Calendar itself.

The "format" attribute dictates what the formatting will look like in the input field itself. I am using %H already that is why when 3 PM is selected 15:00:00 is displayed in the input field.

If I am wrong why is 3 PM correctly being converted into 15:00:00 ?

Working with the uncompressed javascript file
media/system/js/fields/calendar.js

Line 181
} else if (/am/i.test(ampm.value) && hours === 12) {
Needs to be == 12 when doing the comparision

Line 751
pm = (altDate.getHours() > 12);
Needs to be >= 12 due to altDate.getHours() returning [0-23] which makes 12 a pm hour

Line 969
hoursEl.value = hrs;
Do a condition chech of !this.params.time24 and if hrs == 00 hoursEl.value = 12 else hoursEl.value = hrs
if (!this.params.time24) { hoursEl.value = (hrs == "00") ? "12" : hrs; } else { hoursEl.value = hrs; }

Those are the changes I made.
This is my final comment on the issue.

ah i understand you now - sorry for being thick

I will look at making a PR to fix this

Closed see #18240 18240