I've been playing around storing tweets inside mongodb, each object looks like this:

{
"_id" : ObjectId("4c02c58de500fe1be1000005"),
"contributors" : null,
"text" : "Hello world",
"user" : {
    "following" : null,
    "followers_count" : 5,
    "utc_offset" : null,
    "location" : "",
    "profile_text_color" : "000000",
    "friends_count" : 11,
    "profile_link_color" : "0000ff",
    "verified" : false,
    "protected" : false,
    "url" : null,
    "contributors_enabled" : false,
    "created_at" : "Sun May 30 18:47:06 +0000 2010",
    "geo_enabled" : false,
    "profile_sidebar_border_color" : "87bc44",
    "statuses_count" : 13,
    "favourites_count" : 0,
    "description" : "",
    "notifications" : null,
    "profile_background_tile" : false,
    "lang" : "en",
    "id" : 149978111,
    "time_zone" : null,
    "profile_sidebar_fill_color" : "e0ff92"
},
"geo" : null,
"coordinates" : null,
"in_reply_to_user_id" : 149183152,
"place" : null,
"created_at" : "Sun May 30 20:07:35 +0000 2010",
"source" : "web",
"in_reply_to_status_id" : {
    "floatApprox" : 15061797850
},
"truncated" : false,
"favorited" : false,
"id" : {
    "floatApprox" : 15061838001
}

How would I write a query which checks the created_at and finds all objects between 18:47 and 19:00? Do I need to update my documents so the dates are stored in a specific format?