In accrual accounting, you're not meant to book expenses before they happen. The "accrual" part means you recognize (accrue) expenses as they are incurred, which is not necessarily when they are paid (which often happens later). The idea with accrual accounting is to recognize that unpaid liabilities are still a claim against current assets, even if the cash to pay hasn't left the bank account yet.
Under accrual accounting, consider that if you had prepaid future expenses, the prepaid expense amount would actually be recognized in the books as an asset, up until the point where the company actually incurs the expense. Only when the expense gets incurred is it booked as an expense, using up a portion of the prepaid amount.
What ought to be clear from accrual's treatment of prepaid expenses is that you generally don't get to claim as a liability any expense that has not yet been incurred.
That being said – while financial accounting (and the tax man) says you should maintain your books as an accurate historical record, you are still free for your own purposes to make separate "forward looking" projections of your company's income or other metrics. Forward-looking projections for informing decision making is generally what management accounting is concerned with.
Just don't book these fictitious liabilities in your books of record – work in a separate spreadsheet or on a copy of your books. Your actual books should reflect what has actually happened, not what might.
