This morning someone tweeted at me about a problem with the out-of-the-box archive view in Drupal. I have a lengthy post about recreating that view, and the same problem was occurring in my recreated one. If you have a date-based contextual filter -- for example, my /posts-by-date/201301 view where 201301 is the filter value -- and you put a non-date value in that URL, like /posts-by-date/foobar, Drupal throws this error:

Warning: date_timezone_set() expects parameter 1 to be DateTime, boolean given in format_date() (line 2006 of /home/MYSITE/public_html/includes/common.inc).
Warning: date_format() expects parameter 1 to be DateTime, boolean given in format_date() (line 2016 of /home/MYSITE/public_html/includes/common.inc).

This is actually really logical. Your view has been told to look for a date by your contextual filter, and you're giving it a value that isn't a date. The person who tweeted me had it happen with old WordPress URLs, which didn't have the same structure as the ones on the new site.

Fixing this, in the out-of-the-box view or a recreated one, is straightforward:

1. Open your contextual filter. Under "When the filter value is in the URL or a default is provided," check Specify validation criteria.
2. On the validator dropdown that will appear, select Numeric.
3. On the action to take dropdown, select Show "Page not found."

This makes it return a 404 on non-numeric values, which is the behavior we want. Easy enough, but a quick Google search shows that plenty of sites have this problem.

I've also added this information to the original writeup.