The 61-day wash-sale window

The wash-sale window is 61 days long: the 30 days before your loss sale, the sale day itself, and the 30 days after (IRC §1091). Acquire substantially identical securities anywhere in that span — in any of your accounts — and the loss is disallowed to the extent of the replacement shares.

The details people get wrong

• Both boundary days count. A buy exactly 30 days before or exactly 30 days after the sale washes. The first safe re-buy date is day 31 after the sale.
• Calendar days, not trading days. Weekends and holidays count. Sell at a loss on July 5 → the window runs through August 4; August 5 is safe.
• Trade date governs, not settlement date.
• The backward half is real. Buying the dip on June 10 and harvesting an older lot on July 1 is a wash — the June buy sits inside the 30-days-before half. This is also why "buy the replacement first, wait 31 days, then harvest" fails: the sequence only works if the SALE lands more than 30 days after the buy.
• It spans every account: other brokers, your IRAs (permanently, per Rev. Rul. 2008-5), and your spouse's accounts (the cross-account trap).

Example

You sell NVDA at a loss on July 9. The danger zone is June 9 through August 8, inclusive. Any NVDA purchase (or call option on NVDA) in any of your accounts in that span washes the loss pro-rata. The first safe re-buy date is August 9 — which is exactly the per-ticker date our free checker computes from your broker CSVs, across all your accounts, in your browser.