When calculating the interest a bond has earned between coupon payments, how you count the days is crucial. This is especially true in the Indian bond market, where different instruments use different rules. The core of this calculation is the Day Count Fraction (DCF), which represents the portion of a year that has passed during an interest period.

The DCF is a simple ratio: the number of days in the period divided by the total number of days in the year. However, the way you determine both the numerator and the denominator changes depending on the type of bond.

For Indian Sovereign Dated Securities, or G-Secs, the market uses the 30/360 day count convention. This method simplifies calculations by assuming every month has exactly 30 days, and the year has 360 days. It doesn't matter if a month is February (28 days) or August (31 days); for a G-Sec, it's always counted as 30 days.

This convention is a holdover from a time before computers, when making calculations by hand was the norm. Standardizing the length of months made the math much easier.

Let's see how this works. Imagine you hold a G-Sec that pays interest semi-annually on June 15 and December 15. You sell the bond, and it settles on August 22. To calculate the accrued interest owed to you, you need the number of days from the last coupon payment (June 15) to the settlement date (August 22).

Using the 30/360 rule:

• The period is from June 15 to August 22.
• Number of days in June = $30 - 15 = 15$
• Number of days in July = $30$
• Number of days in August = $22$
• Total days = $15 + 30 + 22 = 67$ days.

The Day Count Fraction would be $67 / 360$.

Treasury Bills (T-Bills) and most Corporate Bonds, including Non-Convertible Debentures (NCDs), follow a different rule: the Actual/365 convention. This method is more intuitive. You count the actual number of calendar days in the interest period and divide by a fixed 365-day year.

Let's use the same dates as our G-Sec example for a corporate bond: a period from June 15 to August 22.

Using the Actual/365 rule:

• Number of remaining days in June = $30 - 15 = 15$
• Number of days in July = $31$ (the actual number)
• Number of days in August = $22$
• Total days = $15 + 31 + 22 = 68$ days.

The Day Count Fraction is $68 / 365$.

One extra day might not seem like much, but in large transactions it has a significant financial impact. The choice of convention directly affects the calculation for accrued interest, which in turn affects the bond's final price. Using the wrong convention is a common error that leads to incorrect valuations and settlement amounts.

Let's compare the two conventions for our example period.

As you can see, both the number of days and the resulting fraction are different. All subsequent yield and price calculations depend on getting this first step right. Always confirm the instrument type—G-Sec, T-Bill, or corporate bond—to apply the correct day count convention specific to the Indian market.