Get data as of a specific time

You can use the "as_of_time" attribute in the mql envelope to get data that was present at a specific time.

this query:
{
 "q1" : {
   "as_of_time" : "2007-01-09T22:00:56.0000Z",
   "query" : [
     {
       "domain" : "/architecture",
       "id" : null,
       "return" : "count",
       "timestamp" : null,
       "type" : "/type/type"
     }
   ]
 }
}

returns 12 (there were 12 types in the /architecture domain last january).
{
 "q1": {
   "code": "/api/status/ok",
   "result": [
     12
   ]
 },
 "status": "200 OK",
 "code": "/api/status/ok"
}


this query (no as_of_time attribute):
{
 "q1" : {
   "query" : [
     {
       "domain" : "/architecture",
       "id" : null,
       "return" : "count",
       "timestamp" : null,
       "type" : "/type/type"
     }
   ]
 }
}

returns the current number of types in the architecture domain:
{
 "q1": {
   "code": "/api/status/ok",
   "result": [
     25
   ]
 },
 "status": "200 OK",
 "code": "/api/status/ok"
}

This example was extracted from a response on the developers@freebase mailing list.

One interesting use of this feature is to create a kind of trust authority application. Establish your own trusted view of topics by store the timestamp of a known good version. Then query to get the trusted version.