VALTIME(SEXP) (BOOLEAN)

 Function
  Tests a string for time format validity.

 Syntax
  VALTIME(sexp)

   sexp = Any string expression.

 Return Type & Value
  BOOLEAN
  Returns TRUE if the string is a valid time format, FALSE otherwise.

 Remarks
  PPL does it best to convert incompatible types, as needed,
  automatically.  Converting a STRING type to a TIME type is particularly
  problematic because of the virtually unlimited numbers of strings
  possible.  This function checks to make sure that the hour is from 0 to
  23, the minute is from 0 to 59, and the second (optional) is from 0 to 59.
  Also, each field (hours/minutes/seconds) must be separated by a colon.
  If the string matches these requirements then the string is considered
  valid and TRUE is returned.  Any other string will result in a FALSE
  value being returned.

 Examples
  STRING s
  WHILE (!VALTIME(s)) DO
    INPUT "Time",s
    NEWLINES 2
  ENDWHILE
  TIME t
  LET t = s
  PRINTLN s," ",t