Twitter’s initial public offering could come as early as Nov. 15 if a prediction from PrivCo, a New York research firm specializing in privately held companies, are correct.

PrivCo based the prediction on an expected expiration of Twitter’s lock-up period, the point at which Twitter employees will be allowed to sell their stock. The research firm says that Twitter inadvertently revealed the news in an IPO document filed with the Securities and Exchange Commission.

“Under the terms of the lock-up agreements, beginning February 15, 2014, employees who are not executive officers may be able to sell [their stock],” read a line an early draft of Twitter’s S-1 filing.

From there, all PrivCo had to do was count backwards the usual 90 days required in a lock-up period to determine that Twitter’s IPO date is likely Nov. 15.

“The typical lock-up period for employees to sell Restricted Stock stock to cover tax obligations due upon vesting is typically 90 days after an IPO. In an earlier version of the S-1, IPO advisors slipped up and inadvertently revealed the end-date for the lock-up period as the fixed date of February 15, 2014, rather than leaving the date blank as most filings do until right before the final pre-Roadshow S-1 amendment, when the IPO date is announced,” PrivCo said.