As we have support for intitle, inurl and author, it could be neat to support for search by date.
It would support different key words with different options. For example:

• date keyword that support anything that can be converted to a date according to the php documentation (SQL equivalent: =)
• date-range keyword that support two dates between square brackets with the same format as the date keyword (SQL equivalent: BETWEEN)
• date-list keyword that support a list of individual dates with the same format as the date keyword (SQL equivalent: IN)