The Incomparable Treasury Bill
If you are an investor analyzing corporate bonds, comparing two different bonds is mathematically simple. You look at the annual percentage yield of Bond A, compare it to the annual percentage yield of Bond B, and buy the one that generates the most profit. Both bonds are calculated using the exact same chronological baseline: a standard 365-day year.
However, if you want to compare the profitability of a corporate bond against a United States Treasury Bill (a short-term loan you make to the US Government), the mathematics instantly break down. Treasury Bills do not use a standard 365-day year. By law, the US Government calculates the yield of a Treasury Bill using an artificial 360-day year (often called a “bank year”).
Because the foundational calendars are different, a 5% yield on a Treasury Bill is mathematically incomparable to a 5% yield on a corporate bond. It is like trying to compare kilometers to miles without a conversion formula. To mathematically translate the artificial 360-day yield of a Treasury Bill into a standard 365-day equivalent yield—allowing you to accurately compare it against normal corporate bonds—you must use the TBILLEQ (Treasury Bill Equivalent) function in Microsoft Excel.
Understanding the Syntax
The TBILLEQ function requires only three pieces of data to execute the chronological translation.
=TBILLEQ(settlement, maturity, discount)
- settlement: The exact date you originally purchased the Treasury Bill from the government or on the secondary market.
- maturity: The exact date the Treasury Bill officially expires and the government pays you the face value. (Note: By law, this date must be exactly one calendar year or less from the settlement date. If it is longer than a year, it is a Treasury Note, not a Bill, and this formula will crash.)
- discount: The official, discounted yield percentage initially quoted by the government (based on the artificial 360-day calendar).
Example 1: Translating the Yield
Assume you are a financial analyst deciding where to park $100,000 of corporate cash for six months. You are looking at a highly rated corporate bond that currently offers a guaranteed 4.75% annualized yield.
To see if you can beat that, you look at the government debt market. You find a short-term US Treasury Bill. You plan to purchase it (Settlement) on March 1, 2024. The government promises to pay you back (Maturity) exactly six months later on September 1, 2024.
The government’s official financial portal lists the current discount yield for this specific Treasury Bill at exactly 4.60%. At first glance, the corporate bond (4.75%) appears vastly superior to the government bill (4.60%). But you know the government’s number is distorted by the 360-day calendar rule.
You must translate the 4.60% into a true, 365-day equivalent.
Let’s map out the variables in a spreadsheet:
- Cell A1 (Settlement):
=DATE(2024, 3, 1) - Cell A2 (Maturity):
=DATE(2024, 9, 1) - Cell A3 (Discount):
4.60%
To execute the translation, click on cell B1 and type:
=TBILLEQ(A1, A2, A3)
How this works:
- Excel calculates the exact physical number of days between March 1 and September 1 (184 physical days).
- It analyzes the distorted 4.60% discount rate against the artificial 360-day baseline.
- It algorithmically expands the timeline, translating the profit margin out over a true 365-day calendar year.
- It instantly outputs 0.0483.
If you highlight cell B1 and click the “%” button on the Excel toolbar, it will format the number beautifully as 4.83%.
The Financial Revelation
The TBILLEQ function has completely changed the math. The true, annualized yield of the government Treasury Bill is actually 4.83%, not 4.60%.
Now that the calendars are synchronized, you can compare the two investments fairly. The corporate bond pays 4.75%. The Treasury Bill pays an equivalent of 4.83%. The government debt is mathematically more profitable. By mastering the TBILLEQ function, you gain the ability to strip away chronological distortions and reveal the true comparative value of government debt.