Ex-coupon date is when a bond starts trading without the right to the next coupon payment. If you buy a bond on or after the ex-coupon date, the seller keeps the upcoming coupon even though you'll own the bond when it's paid. In the U.S. Treasury market, the ex-coupon date is typically one business day before the coupon payment date. For corporate bonds, conventions vary (often the record date). Think of it as the dividend ex-date for bonds: just like stocks, there's a cutoff for who gets the payment. After the ex-coupon date, the bond's price typically drops by roughly the coupon amount, reflecting the lost payment. Accrued interest calculations adjust accordingly — accrued may go negative between ex-coupon and payment date.