So, for this limited goal (dimension is time only, and we may require SI units for input, reduced number input formats), we could calculate the quantity into seconds using {convert} module, and enter that number into  to return the sortkey. The visible text returned can be the input I guess. Would not need {val} this way (unless the input number format is not recognised by {convert}. Is {Val} better for this, covering more scientific forms?). -