I am learning a lot looking through all these posts, but can't figure out
this answer. I am creating a spreadsheet of projects with a column for
deadlines. I want to be able to select the deadline from a drop-down
list/data validation.

BUT I want the deadline choices to say things like, "today," "tomorrow," "by
end of week," "in one week," "in two weeks," "in one month," etc. And then
it would automatically input that as an actual "ddd m/d/yy" date.

Also hope to list "Mon." "Tues." etc., where it would input the actual date
of the next upcoming one. And if possible, one for the 1st workday of the
following month, and one for the 15th of the following month (or the Mon
after, if on a weekend). The user should also be able to input a particular
date if they would like. I assume this would be done just by typing it
directly in the cell, right?

This deadline should NOT change as today's date changes. It must remain as
it was originally input, unless it is manually changed.

Oh - and it would be fine if one column had the drop-down box, and the next
column had the actual date. This would help a ton, so I eagerly await any
help you can offer!