However, the underlying search engine using Amazon Cloudsearch syntax. The normal search syntax gets converted to Cloudsearch syntax.
If you use the Cloudsearch syntax, you get access to more information. (in particular, the ability to search by timestamp)
The magic incantations to active Cloudsearch syntax are &syntax=cloudsearch
Threads about this:   
Or you can just use the above search box. For example, try searching for timestamp:1357020000..1357106400
and all search results will be from January 1, 2013.
|selftext:||body of a self-text thread|
|flair_text:||flair attached to the story|
|flair_css_class:||CSS class associated with the flair|
|subreddit:||subreddit it was posted in|
|num_comments:||number of comments|
|timestamp:||Unix time the thread was created|
|nsfw:||is the thread marked NSFW, or not?|
|self:||is the thread a self-thread, or not?|
|downs:||number of downvotes|
|ups:||number of upvotes|
|sr_id:||base36 decoded version of the subreddit ID|
|fullname:||the t3_ fullname of the thread|
|language:||str||find subreddits for a specific language||de, es|
|name:||str||No idea. How is this different from description:, or searching no specific field??|||
|sidebar:||str||Rarely matches. Maybe this was disabled at some point, but it still has some stale data in the inverted index?
(much as this query returns several hits that no longer have 1000 comments?)
|header_title:||str||This one seems to never match.|||