In a thread on the Apple Support Discussions forums, user "Jaymz" reports:
I receive emails for events all the time and they have the following date format (example shown)
Apple Mail used to recognize this as a date and time in the email so I could make a quick iCal addition. This seems to have ceased with the SL upgrade. It still detects more obvious dates and other data but I need it to detect this type as before.
Other users corroborate this information stating that multiple forms of typical date formats are not being recognized by Mail. One Terminal command is suggested in the thread, but does not work properly. Users can attempt to rebuild their Mailboxes (Select the Mailbox and from the Mail menu bar > Mailbox > Rebuild). If the issue persists, drag the "com.apple.mail.plist" file from (username > Library > Preferences) to your Desktop. Reopen Mail and test your messages.
Note: This will delete your Mail settings. If the issue persists and you would like your preferences restored, put the file back into the Preferences folder. If moving the file restores the data detection function, delete the old preferences file from the Desktop (a new one will be created).