The calculator starts with the cash in your bank account today. Each month it adds what comes in and takes out what goes out. The first month the balance would end below zero is the month cash runs short.
Months of cash counts the full months your cash covers, this month included. If money in is equal to or more than money out, cash does not run short, and the chart shows how it builds.
Net cash flow is money in minus money out for the month. When it is negative, that gap is your burn: the amount the bank balance falls each month.
Use cash, not the P&L. A sale you have invoiced but not collected (accounts receivable) is not cash until it lands. Loan payments, owner distributions and equipment purchases are cash out, even though some of them never show up as expenses on the P&L.