The Excel Tbilleq function calculates the bondequivalent yield for a Treasury Bill.
The syntax of the function is:
Where the arguments are as follows:
settlement    The settlement date of the treasury bill (i.e. the date that the bill is purchased). 
maturity    The maturity date of the treasury bill (must be greater than, and within one year of, the settlement date). 
discount    The treasury bill's percentage discount rate. 
Note that the date arguments should be entered into the function as either:
Warning: If you attempt to supply Excel functions with dates in a text form, these can be interpreted differently, depending on the date system and date interpretation settings on your computer.
The following example shows the Excel Tbilleq function used to calculate the bondequivalent yield for a treasury bill with settlement date 01Feb2017, maturity date 30Jun2017 and a discount of 2.5%:
A  B  

1  Settlement Date:  01Feb2017 
2  Maturity Date:  30Jun2017 
3  =TBILLEQ( B1, B2, 2.5% ) 
The above Tbilleq function returns the value 0.025612238, (i.e. 2.56%).
Note that, in the above example:
For further information and examples of the Excel Tbilleq function, see the Microsoft Office website.
If you get an error from the Excel Tbilleq function, this is likely to be one of the following:
#NUM!    Occurs if either:

#VALUE!    Occurs if either:
