The Excel T.DIST.2T function calculates the twotailed Student's T Distribution, which is a continuous probability distribution that is frequently used for testing hypotheses on small sample data sets.
The T.DIST.2T function is new to Excel 2010. However, this function (along with the new T.DIST.RT function) simply replaces the TDIST function, which is available in earlier versions of Excel.
The syntax of the T.DIST.2T function is:
where the function arguments are:
x    The value at which you want to evaluate the twotailed Student's TDistribution. 
degrees_freedom    The number of degrees of freedom (must be ≥ 1). 
Note that the Excel T.Dist.2t Function doesn't allow the argument x to be < 0. Therefore, if you want to calculate the twotailed Student's T Distribution for values of x that are less than 0, you should use the relationship:
The above chart on the right shows the twotailed Student's T Distribution with 10 degrees of freedom.
If you want to calculate the value of this function at x = 1, this can be done using the Excel T.Dist.2t function, as follows:
This gives the result 0.340893132, or 34.09%.
If you want to calculate the value of the function, at x = 1, this must be done using the relationship T.DIST.2T( x, df ) = T.DIST.2T( x, df ).
Therefore, the formula to calculate the function at x = 1 is simply:
which, as shown above, gives the result 0.340893132, or 34.09%.
Further details and examples of the Excel T.Dist.2t function are provided on the Microsoft Office website.
If you get an error from the Excel T.Dist.2t function this is likely to be one of the following:
#NUM!    Occurs if either:

#VALUE!    Occurs if one or both of the supplied arguments are nonnumeric. 