I am looking at some spreadsheets that show the US treasury bonds have some negative accrued. Why would that be the case? Shouldn't bond accruals always be positive?
In some countries/markets other than US, bonds (treasury and corporate too) have an "ex coupon" date some days before the end of the coupon period. The bondholder of record is determined using the ex coupon date, and receives from the bond issuer the full coupon, accrued over the entire coupon period. If he sells the bond settling after the ex coupon date and before the end of the coupon period, then the proceeds are the clean price minus the interest accrued from the settle date to the end of the coupon period.