The quoted solution won't provide the leading zeros you'll need for
single digit hour and minutes. Paul C. has a correct solution, but also
consider the HPHHMMSSMMM variable. Enter :help hphhmmssmmm for more
info. 

This should work:
  setvar newtime hphhmmssmmm
  setvar newtime lft(newtime,2)+":"+str(newtime,3,2)

Note: the above is preferred to this:
  setvar newtime lft(hphhmmssmmm,2)+":"+str(hphhmmssmmm,3,2)

because it is possible that the minutes roll over to the next hour in
the middle of your setvar. For example: 
  setvar newtime lft(hphhmmssmmm,2)+":"+str(hphhmmssmmm,3,2)

Step 1: extract lft(hphhmmssmmm,2)   and you get, say, '08'
Step 2: extract str(hphhmmssmmm,3,2) and you get, say, '00' but at this
instant hphour has rolled to '09'.
Step 3: assign newtime '08:00' but it should be '08:59' or '09:00',
hence you have an ~ 1 hour error.

If you need to use hphour + hpminute then you still should (IMO) deal
with rolled over minutes. E.g.:
  setvar hh hphour
  setvar mm hpminute
  if hh <> hphour then
     setvar hh hphour
  endif
  setvar newtime rht('0!hh',2)+":"+rht('0!mm',2)
  

HTH,
 Jeff Vance, vCSY

...
> Setvar newtime  '!HPHOUR:!HPMINUTE'
...
> The problem I'm trying to solve is to pass a formatted 
> time to another process via the hphour and hpminute vars.  
> For instance hphour = 6 and hpminute = 10, so the formatted 
> string to pass would be 06:10.

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