Typically, CRMs are created to help sales teams plan around deals are won and contracts signed. That's why deals always have "close dates". But often there is a delay between closing a deal and starting to earn revenue from it. That deal "start date" might be delayed for resource or other reasons, but it's quite common that a forecast can't just rely on the close date.
To support this, QuarterOne allows you to map your forecast to a custom date field in your CRM. Once you have created this field:
Log in to QuarterOne and navigate to the assistant menu on the settings page.
Then select the date field that you want to use instead of Close Date from the Start Month drop-down at the top of the page that will look like this....
If you want all of the deal value to be assigned to the month of the new date field, just leave duration as "1 month". Otherwise, you can change this if you want to spread revenue for each deal automatically over a number of months specified in your CRM as described in this article.