In Go you can't define default values for fields, they will always be the zero-value of their type when a new struct value is created (unless you use a composite literal where you can give a different value explicitly).

So one option would be to create a constructor-like function NewUser() which would set this field, and use always this function to create new users:

func NewUser() *User {
    return &User{
        CreatedAt: time.Now(),
    }
}

Of course this can't be forced, and also this will hold the timestamp of the User struct value creation and not when it is saved.

Another, better approach is to use a custom marshaling logic.

You can write custom marshaling logic by implementing bson.Getter. GetBSON() is responsible to provide a value that will actually be saved. We want the same User instance to be saved, just its CreatedAt field set prior:

type User struct {
    CreatedAt time.Time `json:"created_at" bson:"created_at"`
}

func (u *User) GetBSON() (interface{}, error) {
    u.CreatedAt = time.Now()
    type my *User
    return my(u), nil
}

Note that a new my type is created and returned. The reason for this is to avoid stack overflow. Simply returning a value of type *User is bad, because it implements bson.Getter, so GetBSON() would get called endlessly. The new my type does not have this method, so endless "recursion" does not happen (the type keyword creates a new type, and it does not "inherit" methods of the underlying type).

Note that this solution will also overwrite (re-set) the CreatedAt field) even if you just want to re-save a User. So we should add a check whether the CreatedAt field is filled, and only set it if it's the zero value:

func (u *User) GetBSON() (interface{}, error) {
    if u.CreatedAt.IsZero() {
        u.CreatedAt = time.Now()
    }
    type my *User
    return my(u), nil
}

Also see related / similar question: Accesing MongoDB from Go