stock ReturnTime(timevariable)
{
    new milliseconds = timevariable, seconds, minutes, string[20];
    while(milliseconds > 9)
    {
        seconds ++;
        milliseconds = milliseconds - 10;
    }
    while(seconds > 59)
    {
        minutes ++;
        seconds = seconds - 60;
    }
    format(string, sizeof(string), "%d:%02d.%03d", minutes, seconds, milliseconds);
    }
    return string;
}

(3371) : warning 209: function "ReturnTime" should return a value
(3372) : error 010: invalid function or declaration
Pawn compiler 3.2.3664          Copyright (c) 1997-2006, ITB CompuPhase

1 Error.

stock Sec2HMS(secs, &hours, &minutes, &seconds)
{
  if (secs < 0) return false;
  minutes = secs / 60;
  seconds = secs % 60;
  hours = minutes / 60;
  minutes = minutes % 60;
  return 1;
}

new sec = 5162;
new Hours, Minutes, Seconds;
Sec2HMS(sec, Hours, Minutes, Seconds);
printf("%d:%d:%d",Hours, Minutes, Seconds);

//Output: 1:26:2

public OnPlayerDeath(playerid, killerid, reason)
{
        if(IsPlayerConnected(killerid) &&  killerid != INVALID_PLAYER_ID ) { //check if the player is connected and it's a VALID KILL
        if(GetPlayerWantedLevel(killerid) < 6) //if the player's wantedlevel is under 6
        {
        SetPlayerWantedLevel(killerid, GetPlayerWantedLevel(killerid) + 1); //increase it by 1
        }
        Streaks[killerid] ++; //the sign '++' means, increase the variable for the killerid by 1
        GivePlayerMoney(killerid, 1500); //if you like, give the killer some money, else comment this line out
        }
        SetPlayerWantedLevel(playerid, 0); //the player who died will get his wanted level reset to 0
        Streaks[playerid] = 0; //and his streak will end, while giving the value '0' to this variable
        SetPlayerScore(killerid,GetPlayerScore(killerid)+2); //not really necessary, would increase the player's score by 1 point
        GameTextForPlayer(playerid,"~g~ +2 Score",3000,1);
        new str[ 256 ], KillerName[MAX_PLAYER_NAME]; //here, you're defining a string and the killername
        GetPlayerName(killerid, KillerName, sizeof(KillerName)); //receive the information of the killer's name
        switch(Streaks[killerid]) //IMPORTANT: with the function "switch", you're switching / toggling through the killstreaks of a player (Streals). you need the killerid here, because the playerid is the one which is GETTING killed
        {
            case 3:
            {
                format(str, sizeof(str), ""white"["red"KS"white"] "green"%s "grey"is on a kill streak with 3 kills", KillerName);
                SendClientMessageToAll(-1, str);
                GameTextForPlayer(playerid,"REWARD ~n~ ~g~ +3 additional score",3000,6);
                SetPlayerScore(killerid,GetPlayerScore(killerid)+3);
            }
            case 5:
            {
                format(str, sizeof(str), ""white"["red"KS"white"] "green"%s "white"is on a kill streak with 5 kills", KillerName);
                SendClientMessageToAll(-1, str);
                GameTextForPlayer(playerid,"REWARD ~n~ ~g~ +5 additional score",3000,6);
                SetPlayerScore(killerid,GetPlayerScore(killerid)+5);
            }
            case 10:
            {
                format(str, sizeof(str), ""white"["red"KS"white"] "green"%s "white"is on a kill streak with 10 kills", KillerName);
                SendClientMessageToAll(-1, str);
                GameTextForPlayer(playerid,"REWARD ~n~ ~g~ +7 additional score",3000,6);
                SetPlayerScore(killerid,GetPlayerScore(killerid)+7);
            }
            case 15:
            {
                format(str, sizeof(str), ""white"["red"KS"white"] "green"%s "white"is on a kill streak with 15 kills", KillerName);
                SendClientMessageToAll(-1, str);
                GameTextForPlayer(playerid,"REWARD ~n~ ~g~ +10 additional score",3000,6);
                SetPlayerScore(killerid,GetPlayerScore(killerid)+10);
            }
            case 30:
            {
                format(str, sizeof(str), ""white"["red"KS"white"] "green"%s "white"is on a kill streak with 30 kills", KillerName);
                SendClientMessageToAll(-1, str);
                GameTextForPlayer(playerid,"REWARD ~n~ ~g~ +15 additional score",3000,6);
                SetPlayerScore(killerid,GetPlayerScore(killerid)+15);
            }
            case 50:
            {
                format(str, sizeof(str), ""white"["red"KS"white"] "green"%s "white"is on a kill streak with 50 kills", KillerName);
                SendClientMessageToAll(-1, str);
                GameTextForPlayer(playerid,"REWARD ~n~ ~g~ +20 additional score",3000,6);
                SetPlayerScore(killerid,GetPlayerScore(killerid)+20);
            }
            }
        return 1;
}