Should date by included in subject Line of webinar invite email?

I personally reccommend the following for any events, because this is how I would think as my target:

• Date, Time because i need to make a quick decision to read further. If I'm not around anyway, why bother?
• Title of Webinar/Catchy thing + Date + Time
• put reg button toward top.
• make subject line consistent with whatever is in top third of email, since people preview.

Second, Open rates are unreliable and only directional. Worry more about CTR and actual form fill outs.

Hi Maria

Agree with Josh, as including date/time in the email would not only ensure quick decision, it would result in more authentic clicks and there would be more conversions (Attendance). 

Date/Time is the first thing anyone would like to know about any event. If you provide all the relevant details in one email there are greater chances of conversion/ registration. As a practice, Email copy should contain Date, Time, Venue, Event details and Speaker details. Beneath email copy, Registration link should be provided. Title should come in the Top Banner of the email. Subject line should give a hint about the webinar or agenda thereby increasing reader's curiousity about moving further and open the email.

Open rates definitely give you directions analyzing which you can plan sending Invites.

However, here're a couple of suggestions for CTR,
- Duration of the event can be included in subject line
- Personalization: usage of first name in email rather than using no name
- Add social sharing buttons/forward to friend options

If CTR is what you're looking for, adding something like "Topic - Next Week" with "Speaker" would be more.

Here's another checklist for best practices for subject lines. 
http://www.marketingexperiments.com/blog/research-topics/email-marketing/subject-line-checklist.html

Hope this helps!