What a leap day does under Act/360

The sample February is 2026, which is not a leap year. Invent a 29 February on the March opening balance at the March opening rate, so the extra day is visible against figures already in hand. $1,847,220 × 0.0697 × 1 ÷ 360 = $357.642316…, which prints as $357.64. That is the leap day. It is also the ordinary daily addend from lesson 9. Act/360 does not treat 29 February as a special fraction. It is one more actual day.

A 28-day February on this balance at 6.97 percent is $10,013.98. A 29-day February is $1,847,220 × 0.0697 × 29 ÷ 360 = $10,371.627183…, which prints as $10,371.63. The single-product 29-day figure is one cent above $10,013.98 + $357.64, which is the rounding split from lesson 14. The economic object is one extra daily addend.

30/360 does not see the leap day. It assigns thirty days to February whether the calendar has 28 or 29. On this balance at 6.97 percent a 30/360 February is $1,847,220 × 0.0697 × 30 ÷ 360 = $10,729.27, the same product lesson 21 uses for a 30-day March. Under 30/360 a leap year and a common year charge February identically. Under Act/360 they differ by $357.64.

The leap day also lengthens the year. A 366-day Act/360 year at a constant 6.97 percent on $1,847,220 is $1,847,220 × 0.0697 × 366 ÷ 360 = $130,897.09, against $130,539.45 for 365 days. The extra $357.64 is again the same daily addend. The convention that produces a larger charge than a 365-day divisor produces a still larger one when the calendar itself is longer.