db.mycollection.find({ _id: { $gt: objectIdWithTimestamp('1980/05/25') } });

gistfile1.js

// This function returns an ObjectId embedded with a given datetime
// Accepts both Date object and string input

function objectIdWithTimestamp(timestamp)
{
    // Convert string date to Date object (otherwise assume timestamp is a date)
    if (typeof(timestamp) == 'string') {
        timestamp = new Date(timestamp);
    }

    // Convert date object to hex seconds since Unix epoch
    var hexSeconds = Math.floor(timestamp/1000).toString(16);

    // Create an ObjectId with that hex timestamp
    var constructedObjectId = ObjectId(hexSeconds + "0000000000000000");

    return constructedObjectId
}


// Find all documents created after midnight on May 25th, 1980
db.mycollection.find({ _id: { $gt: objectIdWithTimestamp('1980/05/25') } });