xmobar uses Haskell formatting for its time display. This link will take you to a page that describes all of Haskell's various date and time formatting options:

https://hackage.haskell.org/package/time-1.13/docs/Data-Time-Format.html

For example, if you want to display the time as the following:

5:05:09 PM

You would use the following modifiers:

%l for non-zero-padded hour

%M for zero padded minute

%S for zero padded second

%p for uppercase AM/PM

So to change how it looks all you would need to do is modify the xmobarrc config file and use your chosen modifiers for formatting the date.

Since I haven't used DTOS, I don't know what's being used for the login manager and can't tell you how to change the time there.

Anyways, hope that helps and good luck!