The backtesting feature helps you stretch your portfolio back while taking into account various inception dates of the funds in your portfolio.
You can start on your existing portfolio, or create a new one (if you want to edit your existing portfolio before backtesting, you can duplicate it first to preserve the current version). Just add all positions with desired weights, and pick this month as investment date for all of them.
After you're done with picking positions, click on the Backtest button on the top right
Create a New Portfolio option will create the backtested portfolio while preserving the current portfolio. Use this option, if you backtest an existing portfolio and don't want to lose current data.
Modify Existing Portfolio option will overwrite the current portfolio with the backtested version. Only use this option if you backtest a new portfolio that you created just for the purpose of backtesting.
You can choose the portfolio inception date by specifying the percentage of positions that need to have track record for the portfolio to begin. You can also supply an exact date.
The rebalancing schedule, will determine, how often will the portfolio get rebalanced back to its original weights.
Positions that are not yet available will have their weight proportionally redistributed among existing positions, until their track record begins.