Format chrono: implemented glibc extension for padding seconds, minutes and hours

[LukashonakV]

Dear @ShawnZhong ,
Thank you for your great PR #2959, #3271 .
For simple sys_time time points it works well.
Is it possible to extend this functionality on the whole chrono.. for example for all time zone API which is included into C++20 standard. ?
The issue here: waybar uses date::zoned_time class which is std::chrono::zoned_time (See zoned_time) .. and for this TZ API padding seconds, minutes, hours doesn't work :(

Here an example of waybar code: clock.cpp:166 . Then when we try to get formatted output at clock.cpp:178 we got wrong printed result. In the same time all other common format masks work well with zoned_time.

[vitaut]

This can be done with a custom formatter but a PR to support zoned time would be welcome if the extension makes sense there.

[bradrn]

a PR to support zoned time would be welcome if the extension makes sense there.

I see no reason why it shouldn’t make sense there — if the time can be printed with a leading 0, it can surely be printed with a leading space too! If I get time I might look into making a PR, but until then perhaps this issue should be re-opened.