Thunderbird

Pretty easy, once you know where to look-- not under Calendar, but under File > New > Calendar

then On the Network and (probably) iCal/ICS.

To add a date and time to the quote of messages you are replying to:

1. Go to Preferences (on a Mac, at least, and this will be under the main Thunderbird menu)
2. Click on the Advanced icon
3. Stay on the General tab and click the Config Editor... button.
4. Search for reply_header_type
5. Double-click the value field of mailnews.reply_header_type, and
6. Change this from 1 (in my case) to 2.

You're done!