Mongoose Date Subtract error

var findLeastHistoryArticlePipeline = [{
        '$group': {
            '_id': { title: '$title' },
            maxTime: { $max: '$timestamp' },
            minTime: { $min: '$timestamp' }
        }
    },
    {
        '$project': {
            '_id': 0,
            'title': '$_id.title',
            'age': { $subtract: ['$maxTime', '$minTime'] }
        }
    },
    {
        '$sort': { 'age': 1 }
    },
    { '$limit': 1 }
]

I got this aggregation pipeline for my mongodb query. But the return result shows that an error

assert: command failed: { "ok" : 0, "errmsg" : "cant $subtract aString from a String", "code" : 16556 } : aggregate failed

It seems that the maxTime and minTime are regarded as String type.

I've tried use new Date('$maxTime') but the result turns out to be 0 all the time.

Any suggestions?

via lhan