In the Windows 7 search box, keywords are case-insensitive, but Boolean operators are case-sensitive. Typing
System.FileName:~=".doc" AND datemodified:1/5/2016
works fine. Typing
System.FileName:~=".doc" and datemodified:1/5/2016
doesn’t given an error message or a warning — it just burns a lot of disk searching and finding nothing. Live and learn!