Merge two MongoDB databases from old driver to official driver

Question

I am using go and I need to merge these two databases to keep the clients happy. I have changed the go code to use the official driver but it refuses to accept time.Time.
So in short, I need to transfer this model database

MgoResults struct {
        ID             bson.ObjectId  `bson:"_id"`
        ZipID          string         `bson:"zid"`
        Message        string         `bson:"message"`
        Result         string         `bson:"res"`
        Error          error          `bson:"errors"`
        ExpirationDate time.Time      `bson:"expirationDate"`
    }

into this model database

MgoResults struct {
        ID             primitive.ObjectID  `bson:"_id"`
        ZipID          string              `bson:"zid"`
        Message        string              `bson:"message"`
        Result         string              `bson:"res"`
        Error          error               `bson:"errors"`
        ExpirationDate primitive.DateTime  `bson:"expirationDate"`
    }

I don't have any strategies yet, but I welcome any examples with go code. I have thought about iterating through all items and changing types then saving it to new model. Both databases work perfectly.

My main problem is during decoding and rebuilding the struct. I am probably doing something wrong.

    var v bson.M
    err = result.Decode(&v)

    if err != nil {
        log.Println("error at decode result : ", err)
    }
    //start filling the struct
    res.ZipID = v["zid"].(string)
    if v["errors"] == nil{
        res.Error = nil
    }else{
        res.Error = v["errors"].(error)
    }

    res.Message= v["message"].(string)
    res.ID      = v["_id"].(primitive.ObjectID)
    res.Result  = v["res"].(string)
    if v["expirationDate"] == nil{
        //res.ExpirationDate = time.Now()
    }else{
        res.ExpirationDate = v["expirationDate"].(time.Time )
        //res.ExpirationDate = v["expirationDate"].(primitive.DateTime)
    }

Answer 1

Thanks to @icza and this slightly related answer here: https://stackoverflow.com/a/64419437/2912740 .

I was able to modify my code a bit and it worked. So it looks like mongo returns its own time that needs some conversion to regular time.

mytime := v["expirationDate"].(primitive.DateTime)
res.ExpirationDate = mytime.Time()