For measuring the interest earned on municipal bonds and corporate bonds, we use the 30 days per month convention or 30/360 convention.

In all-day count conventions, the last day is always excluded. In this case, we are interested in the number of days between September 1st and February 12th.

Therefore, we will assume that the months of Sept, Oct, Nov, Dec, and Jan have 30 days each and then add 11 days in Feb. That gives a total of 161 days.

Days between September 1st and February 12th (30 days convention) = 161 days
Days between September 1st and March 1st (30 days convention) = 180 days

Therefore, the interest earned is:

$`$6.5`% \times \frac{161}{180} = 5.81%$$

Note: For measuring the interest earned on Treasury bonds, we use the actual number of days between the dates/actual number of days between reference periods. For measuring interests earned on money market instruments, we use the actual days divided by 360 days per year convention (and, in some countries, 365 days per year is also used).