clamp
Description:
If this is prior to min_date, sets this equal to 
        min_date.
 If this falls after max_date, sets this equal to 
        max_date. Otherwise, this is unchanged. Either of min_date and max_date may
       be null. All non-null dates must be valid.
Parameters:
| this | 
           a Date to clamp  | 
      
| min_date | 
           minimum accepted value for this  | 
      
| max_date | 
           maximum accepted value for this  |