To add the date and time to a Send Quote message automatically, you can use Personalization Tokens (learn more here)

First, edit your Send Quote template (Article) or create a new template (Article). While editing or creating your template, you can insert the "QuoteExpirationDate" or "QuoteExpirationTime" personalization tokens.

When you insert a template with these Personalization Tokens, the correct information is pulled from the project - so you don't have to type it yourself!