hello community

can anybody in the community please tell me what is this square bracket with
dollar symbol follow with a dash inclusive the number "409" at the front the
the custom format.........example:

[$-409]h:mm:ss AM/PM;@.......

when I wanted to make the "AM/PM" to "a/p", I need to delete away the
"[$-409]" and replace the "AM/PM" with "a/p", then it works.
however
if the do not delete the [$-409] away, the "AM/PM" would not be replace by
"a/p"...why is it so ?

thanks everybody for the kind explanation :)
--
oldLearner57