Two returns can both be right
A fund or benchmark return ignores when you added money. Your own result does not: a large deposit just before a rise helps, and a large deposit just before a fall hurts. Morningstar measures this gap between fund returns and investor returns every year in its Mind the Gap study.
A broker app usually shows one account return. The contribution timing review isolates the part that came from the dates of your deposits and withdrawals, and shows it in money.
Build a unit price for your own portfolio
First remove the effect of cash flows from your portfolio’s growth. Start a unit price at 1 on the first day. From one valued day to the next, multiply it by the account value before that day’s deposits and withdrawals divided by the previous value. This is the time-weighted index of your own holdings.
With that unit price, every deposit can be followed to today: its value now is the amount times today’s unit price divided by the unit price on the deposit date. Summed over all deposits and withdrawals, this equals the actual account value, so the comparison uses the same growth on both sides.
Compare with spreading the same money evenly
Take your net contribution, deposits minus withdrawals, and spread it in equal parts over every valued trading day from the first cash flow to the last. Grow each part with the same unit price. The benchmark is not used: the comparison is your own portfolio with a different schedule.
Actual value minus the even-schedule value is the result. Positive means the dates you chose did better than steady contributions. Each deposit and withdrawal also gets its own share of the difference, and a withdrawal before a rise counts against you.
Worked example: two deposits
You deposit $10,000 when the unit price is 1.00 and another $10,000 after a 25% rise, at 1.25. Today the unit price is 1.50. The first deposit is worth $15,000 and the second $12,000, so the account is $27,000.
For a simplified even schedule, split the $20,000 over four dates with unit prices of 1.00, 1.08, 1.16, and 1.25. Each $5,000 grows by 1.50 divided by its price, which averages about 1.3455, so the even schedule reaches about $26,910. Your timing added about $90. Portle uses every valued trading day rather than four, but the method is the same.
When the review cannot give a number
The result is not shown as zero when it cannot be measured. There may be no deposits or withdrawals, cash flows on only one day, more money withdrawn than deposited, or a day when a deposit is larger than the account value that day, which means the records do not agree with each other.
Days without a known value are carried to the next valued day. If price history starts after your ledger does, earlier cash flows are treated as made on the first day of history, and Portle says so.
Where to see it in Portle
The contribution timing review is a free card next to the sell and buy timing reviews. Together they show how much the dates you sold, bought, deposited, and withdrew changed your result.
It uses the same performance series as the performance card, so no extra data is requested. The detail page lists each deposit and withdrawal with its share of the difference.