This is to reflect the change in accrued interest for your bond holding during the selected period.

• Assuming you have selected 1/12/2018 as start date of period and 14/12/2018 as end date of period

• Last coupon date is 06/06/2018 and next coupon date is 06/12/2018

• Accrued interest of bond on start date is 178 days

• Accrued interest of bond on end date is 8 days

• Hence the negative amount (8 days – 178 days)