You can set the portfolio date when you are first building/importing a portfolio. From the "Portfolios" section, click on "Create".
On the next screen, at the top, you can change the portfolio date.
Now, if you want to change the date of an existing portfolio, please do the following. From the "Portfolios" section, click on the portfolio you want to edit.
Now click on "Edit", at the top.
Change the date then hit "Save".
Setting the date enables portfolios to be uploaded at historical points in time. The calculation engine will use market data prevailing at that point in time.