Gmail sometimes allows me to add an event (from the email body) to my Google Calendar:

This is a very handy feature. The problem is - sometimes it doesn't show up, even though there is clearly a specific date in the email body:

Gmail seems to parse the email for any occurrences of dates, but quite often it misses obvious ones.
So why don't I get the option to add to my calendar, for this email? It saves me a lot of steps.
The behaviour appears to be somewhat random. For the SAME email, my buddy gets the "add to calendar" option and I don't. I've messed around with my settings but nothing seems to work.