\starttext

\unprotect \pushoverloadmode

\setnewconstant \plussixty      60
\setnewconstant \plustwentyfour 24

\permanent\tolerant\protected\def\time[#1]%
  {\ifarguments
     \expandafter\normaltime
   \or
     \begingroup
     \resetdummyparameter h%
     \resetdummyparameter m%
     \ifempty{\dummyparameter h\dummyparameter m}%
       \setdummyparameter h{\normaltime/\plussixty}%
       \setdummyparameter m{\normaltime;\plussixty}%
     \else
       \letdummyparameter h\zerocount
       \letdummyparameter m\zerocount
     \fi
     \setdummyparameter\c!separator{:}%
     \getdummyparameters[#1]%
     \scratchcounterone{\dummyparameter h}%
     \scratchcountertwo{\dummyparameter m}%
     \advanceby\scratchcounterone{\scratchcountertwo/\plussixty}%
     \scratchcountertwo{\scratchcountertwo;\plussixty}%
     \scratchcounterone{\scratchcounterone;\plustwentyfour}%
     \ifnum\scratchcounterone<\plusten0\fi\tointeger\scratchcounterone
     \dummyparameter\c!separator
     \ifnum\scratchcountertwo<\plusten0\fi\tointeger\scratchcountertwo
     \endgroup
   \fi}

\popoverloadmode \protect

todo:

\startitemize
\startitem either of not leading zero \stopitem
\startitem AM/PM but where and is this language dependent \stopitem
\stopitemize

\startlines
primitive: \the\time
test: \time[m=40] \time[m=100] \time[]
\stoplines

\dorecurse{100}{
    \dorecurse{200}{
        \time[h=#1,m=##1]
    }
    \par
}

\stoptext