[Ruben_Alonso]

It will depend on the system that you made, for example

Normaly on gamemodes they do:

pawn Код:

enum pInfo
{
    pDateRegistered[11]
};
new PlayerInfo[MAX_PLAYERS][pInfo];

//then you add a command, if you have zcmd

CMD:dateregistered(playerid, params[])
{
    new id, string[64];
    if(sscanf(params, "u", id)) return SendClientMessage(playerid, -1, "/dateregistered <playerid>.");
    else if(id == INVALID_PLAYER_ID) return SendClientMessage(playerid, -1, "Type a valid user ID.");
    format(string, sizeof(string), "He registered at %s", PlayerInfo[id][pDateRegistered]);
    SendClientMessage(playerid, -1, string);
    return true;
}

I've just made a simple command for you to understand, you must go in detail if you'd like to see it better.