I want to create a SharePoint Date and time Column, so that the user is only allowed only to select a date 20 days ahead of today's date.
You could create a Calculated Column like this. You'll notice I've referenced the default [Modified] date field, you can substitute this for an alternative date field. Using the [Created] date field might work better for you or else make a new date field.
Back to my example - the only trouble is if someone changes the item the [Modified] date will change and the calculated column will also recalculate!
If you could give more detail in your question, then I'd see whether this would work for you or not.