You can create expenses that don’t yet have any payment recorded against them. These will show as money owed to your supplier on your expenses dashboard. However, the date you’re raising it for would have to be either that day’s date or earlier. It’s not possible to create an expense with a date in the future.
One way you could do what it sounds like you’re looking for would be to create your expenses so the line item is dated for the beginning of the month. Then, once you’ve actually made the payment, you can update the expense with the amount you paid on the date it was actually paid.
This article may help: What are unpaid expenses and what to do about them
Hope this helps,