In macOS, the date and time formatting strings have a UI which allows editing them by bringing tokens in and out of the format string edit fields. The field is also editable so constant strings can be intertwined with the tokens which will become substitued with the actual variable value.

This seems to interact weirdly with the System Preferences > Languages & Region > General > 24-Hour Time setting. Specifically, in case this setting is ON (so the AM/PM token should not be included in the time format string) while the AM/PM box is included in the time format string at the same time, it will remain a part of it, when it should be removed instead.

Also, in the case of this toggle being OFF (so the token should be included in the time format string) while at the same time the time format field already including the AM/PM token, if the token is enclosed in constant string characters, another one will be appeneded to the final format string. If it is enclosed by other tokens or white-space, it will be moved to have a space in front of it instead.

This is all very weird, but the main concern is that in case the AM/PM token is included in the time format string and the 24-Hour Time toggle being ON, the format string will still include the a token instead of not doing so!

This token is included by default, and when using the default, it will be removed or added depending on the value of the toggle, but if changed, the toggle will lose its ability to correctly manipulate these format strings wrecking everything holy.

make

There are two options for configuring date and time in macOS:

There are these two settings:

• Use a 24-hour clock
• Show AM/PM

These settings only apply to the menu bar!

There is the 24-Hour Time setting.

Whether or not the a token (for AM/PM) is present in the format string is then determined according to the following rules:

I have enclosed the AM/PM token in parentheses when added manually in the time format fields to differentiate them from when the OS adds them due to the 24 hour format switch being off and them not being in the pattern.

time short: "HH-mm(a)"
time medium: "HH-mm-ss(a)"
time long: "HH-mm-ss(a)z"
time full: "HH:mm:ss(a)zzzz"
        

time short: "h-mm(a) a"
time medium: "h-mm-ss(a) a"
time long: "h-mm-ss(a)z"
time full: "h:mm:ss(a)zzzz"
        

time short: "HH-mm"
time medium: "HH-mm-ss"
time long: "HH-mm-ssz"
time full: "HH:mm:sszzzz"
        

time short: "h-mm a"
time medium: "h-mm-ss a"
time long: "h-mm-ssz"
time full: "h:mm:sszzzz"
        

• Without the parentheses (might this have an effect on whether the token gets preserved or appended?) In this case the AM/PM token got moved instead of appended, which is a better result:

time short: "HH-mma"
time medium: "HH-mm-ssa"
time long: "HH-mm-ssaz"
time full: "HH:mm:ssazzzz"
        

time short: "h-mm a"
time medium: "h-mm-ss a"
time long: "h-mm-ssaz"
time full: "h:mm:ssazzzz"
        

time short: "HH-mm"
time medium: "HH-mm-ss"
time long: "HH-mm-ssz"
time full: "HH:mm:sszzzz"
        

time short: "h-mm a"
time medium: "h-mm-ss a"
time long: "h-mm-ssz"
time full: "h:mm:sszzzz"