_formatNumberToString(num, prependZero) {
      return num < 10 && prependZero ? `0${num}` : num.toString();
    }
Return a value as string, formatted and prepended with zero if necessary.

@param {number} num A number to format
@param {boolean} prependZero Whether to prepend with zero or not
@param {number} max Max value of the number for this clock
@returns {string} Formatted value
@private