ERC20 Tokens - Decimals and resell

Question

I have just created an ERC20 Token; i set the decimal to 4 and bonus value to 1200 then i sent 0.3 ether to its contract address to purchase my new tokens. I received 36 quadrillion tokens in return and i don't understand why! before ETH network i have created a same contract at ROPSTEN testnet but with 18 decimals and i got 360 Tokens in return of 0.3 ether

I have these two questions: (a) what is the math here why i got quadrillions at network ETH? (b) can i convert tokens back to ether as a sender or as a contract owner? if yes how?

i have also tried sending tokens from myetherwallet but it is showing 0 of my tokens but i can see tokens under my address at both etherscan and ethplorer.io

contract code:

    function constructorfunc() public {
        symbol = "TKN";
        name = "token name";
        decimals = 4;
        bonusEnds = now + 1 weeks;
        endDate = now + 7 weeks;

    }



  function () public payable {
        require(now >= startDate && now <= endDate);
        uint tokens;
        if (now <= bonusEnds) {
            tokens = msg.value * 1200;
        } else {
            tokens = msg.value * 1000;
        }
        balances[msg.sender] = safeAdd(balances[msg.sender], tokens);
        _totalSupply = safeAdd(_totalSupply, tokens);
        Transfer(address(0), msg.sender, tokens);
        owner.transfer(msg.value);
    }

other functions that i have added in my contract

// ------------------------------------------------------------------------
    // Total supply
    // ------------------------------------------------------------------------
    function totalSupply() public constant returns (uint) {
        return _totalSupply  - balances[address(0)];
    }


    // ------------------------------------------------------------------------
    // Get the token balance for account `tokenOwner`
    // ------------------------------------------------------------------------
    function balanceOf(address tokenOwner) public constant returns (uint balance) {
        return balances[tokenOwner];
    }


    // ------------------------------------------------------------------------
    // Transfer the balance from token owner's account to `to` account
    // - Owner's account must have sufficient balance to transfer
    // - 0 value transfers are allowed
    // ------------------------------------------------------------------------
    function transfer(address to, uint tokens) public returns (bool success) {
        balances[msg.sender] = safeSub(balances[msg.sender], tokens);
        balances[to] = safeAdd(balances[to], tokens);
        Transfer(msg.sender, to, tokens);
        return true;
    }


    // ------------------------------------------------------------------------
    // Token owner can approve for `spender` to transferFrom(...) `tokens`
    // from the token owner's account
    //
    // https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20-token-standard.md
    // recommends that there are no checks for the approval double-spend attack
    // as this should be implemented in user interfaces
    // ------------------------------------------------------------------------
    function approve(address spender, uint tokens) public returns (bool success) {
        allowed[msg.sender][spender] = tokens;
        Approval(msg.sender, spender, tokens);
        return true;
    }


    // ------------------------------------------------------------------------
    // Transfer `tokens` from the `from` account to the `to` account
    //
    // The calling account must already have sufficient tokens approve(...)-d
    // for spending from the `from` account and
    // - From account must have sufficient balance to transfer
    // - Spender must have sufficient allowance to transfer
    // - 0 value transfers are allowed
    // ------------------------------------------------------------------------
    function transferFrom(address from, address to, uint tokens) public returns (bool success) {
        balances[from] = safeSub(balances[from], tokens);
        allowed[from][msg.sender] = safeSub(allowed[from][msg.sender], tokens);
        balances[to] = safeAdd(balances[to], tokens);
        Transfer(from, to, tokens);
        return true;
    }


    // ------------------------------------------------------------------------
    // Returns the amount of tokens approved by the owner that can be
    // transferred to the spender's account
    // ------------------------------------------------------------------------
    function allowance(address tokenOwner, address spender) public constant returns (uint remaining) {
        return allowed[tokenOwner][spender];
    }


    // ------------------------------------------------------------------------
    // Token owner can approve for `spender` to transferFrom(...) `tokens`
    // from the token owner's account. The `spender` contract function
    // `receiveApproval(...)` is then executed
    // ------------------------------------------------------------------------
    function approveAndCall(address spender, uint tokens, bytes data) public returns (bool success) {
        allowed[msg.sender][spender] = tokens;
        Approval(msg.sender, spender, tokens);
        ApproveAndCallFallBack(spender).receiveApproval(msg.sender, tokens, this, data);
        return true;
    }

Answer 1

what is the math here why i got quadrillions?

if (now <= bonusEnds) {
  tokens = msg.value * 1200;
} else {
  tokens = msg.value * 1000;
}

msg.value is in wei. By sending 0.3 ether, you credited yourself with 300000000000000000 * 1200 tokens.

can i convert tokens back to ether as a sender or as a contract owner?

You can do both, as long as the contract has enough ETH I suppose. You can include a simple function, for the owner only or not, which can be called and will convert X tokens into Y ether for a converting rate you define I suppose.