Both dates, one day
Purja stores the AD date as the database value and returns the corresponding Bikram Sambat date beside it. A bill, payment, stock movement and report all refer to the same day. The screen can show Nepali or English, and printed bills can carry both.
Why July 16 is not the rule
Shrawan 1 does not fall on a fixed AD date every year. A shortcut that always starts the fiscal year on July 16 eventually puts a day in the wrong year. Purja uses an embedded, checked month table to find the actual Shrawan 1 for each year.
The fiscal year is part of the invoice number
A number such as INV-083/84-00042 says which Bikram Sambat fiscal year issued it. Drafts have no number. The server assigns the next number when the invoice is issued, and the counter starts again in the next fiscal year.
Dates in reports
Choose a Bikram Sambat period and Purja resolves the exact AD boundaries before reading the ledger. That keeps the sales book, purchase book, VAT figures and ordinary reports on the same period.
The simple rule: AD is stored truth. Bikram Sambat is derived from the real month table and returned by the server.