The value doesn't update automatically, like a ticking clock. It updates whenever the macro call is rendered, such as when the containing tiddler's display needs to be refreshed for some other reason.
0hh:0mm, DDth MMM YYYY
Note: The format string
[UTC]YYYY0MM0DD0hh0mm0ssXXX will return a date string representing the UTC time-stamp as it is used in the TiddlyWiki
modified time-stamp fields.