I have a free open-source add-in that can do that for you.
For example, these two formulas:

=RCHGetElementNumber("CSCO",1290)
=RCHGetElementNumber("CSCO",1295)

...return:

8-May-07
6-Feb-07

...from earnings.com. There is a template in the files area to return
the last 23 earnings announcements from earnings.com.