How to calculate Month-to-date unrealized and Realized P&L

Mark-to-market accounting for tax purposes would adjust unrealized cost basis for the beginning of the new tax year. This situation applies to futures contracts and to trader-status traders and reflects tax owed on both realized and unrealized gains at year-end.

But mark-to-market accounting simply for the purpose of combining realized and unrealized gain/loss together, to make a portfolio viewpoint or a liquidity-value, would not adjust cost basis.

For instance if the year-to-date, or portfolio-to-date, value was 10000 at May 30 but 9500 at June 15 then the month-to-date gain/loss, or profit/loss, at June 15 was -500. Careful accounting of deposit/withdrawal is required. Deposit is not gain and withdrawal is not loss.

(The software that I use says that a cost basis can be adjusted with a return-of-capital transaction along with a balancing deposit or withdrawal. And then the return-of-capital transaction has five different uses. In other words, there is a minimum set of financial operations that when combined together can perform all financial operations. There is no cost-basis-adjustment transaction but just the combination of return-of-capital with deposit or withdrawal.)

You don't need expensive complicated software to calculate month-to-date unrealized and realized performance.

If you want aggregate performance, use mark-to-market totals (MTM), making sure to adjust for cash additions or withdrawals as well as unpaid dividends, if any.

For individual performance per position, segregate your open and closed positions. For each open position, compare the closing value at the end of last month and the closing value of the position on the subsequent date. Use the buy date if a position was taken after the last date of last month. For closed positions, use the sale date.

This is very easy to set up in a spreadsheet. All you need to figure out is what stats you want to calculate and display.