Return datetime string for use with time attributes.

        Handling depends on input:
          'now'   - returns datetime for now
          number  - assume datetime values, generate string
          other   - no change, return same value

def _str_datetime_now(self, x=None):
        """Return datetime string for use with time attributes.

        Handling depends on input:
          'now'   - returns datetime for now
          number  - assume datetime values, generate string
          other   - no change, return same value
        """
        if (x == 'now'):
            # Now, this is wht datetime_to_str() with no arg gives
            return(datetime_to_str())
        try:
            # Test for number
            junk = x + 0.0
            return datetime_to_str(x)
        except TypeError:
            # Didn't look like a number, treat as string
            return x