new bool:TimeChanged[MAX_PLAYERS];

public OnPlayerConnect(playerid){
    TimeChanged[playerid] = false;
    return 1;
}
public OnPlayerCommandText(playerid, cmdtext[]) {
    if(!strcmp(cmdtext,"/night",true)) {
        if(!TimeChanged[playerid]) {
            SetWorldTime(0);
            TimeChanged[playerid] = true;
            return SendClientMessage(playerid, 0xFFFFFFFF, "SERVER: Night is ON.");
        }
        else return SendClientMessage(playerid,0xFFFFFFFF,"SERVER: You can change that only once.");
    }
    if(!strcmp(cmdtext, "/day", true)) {
        if(TimeChanged[playerid]) {
            SetWorldTime(12);
            TimeChanged[playerid] = false;
            return SendClientMessage(playerid, 0xFFFFFFFF, "SERVER: Day is ON.");
        }
        else return SendClientMessage(playerid,0xFFFFFFFF,"SERVER: You can change that only once.");
    }
    return 0;
}