The Excel Floor.Math function is one of fifteen rounding functions in Excel.
A full list, along with comparison tables, is provided on the Excel Rounding Functions page.The Excel Floor.Math function rounds a supplied number down to a supplied multiple of significance.
In general, positive values are rounded towards zero and negative numbers are rounded away from zero. However, the user can reverse the direction of the rounding for negative numbers.
Note: the Floor.Math function was first introduced in Excel 2013 and so is not available in earlier versions of Excel.
The syntax of the function is:
Where the arguments are:
number    The number that is to be rounded down. 
[significance]   
An optional argument that specifies the multiple of significance to round the supplied number to. If omitted, the [significance] argument takes on the default value 1 (i.e. round down to the nearest integer). (Note that the arithmetic sign of the [significance] argument is ignored). 
[mode]   
An optional numeric argument that reverses the direction of rounding for negative numbers only.

In column B of the following spreadsheet, the Excel Floor.Math function is used to round down the positive and negative numbers 58.55 and 58.55, to different significance values.
Formulas:

Results:

The above examples show how:
For further information and examples of the Excel Floor.Math function, see the Microsoft Office website.
If you get an error from the Excel Floor.Math function, this is likely to be the #VALUE! error:
#VALUE!    Occurs if any of the supplied arguments are nonnumeric. 