Of what i remember use the Int() function. ex

int(2.99) = 2 ; int(2.1)=2

and so on.

Replace Floor() with Int(). I learned this by searching in the Access help files, in this case, hitting F1 while in the query designer, and searching for "functions." That took me to a help topic comparing VBA and T-SQL functions.

You should probably have a look at the Access database engine SQL Reference. I can't find a good online reference for functions that are supported through the Jet/ACE and Access expression services. For some unknown reason, the Access Help has not included Jet/ACE expressions since Jet 3.0 and this aged resource was finally removed from MSDN a year or two ago :(

Keep in mind that the Jet/ACE expression service for use outside Access supports a much smaller subset of functions that is possible using the Access Expression Service when running your SQL inside Access 2007. Broadly speaking, the VBA5 functions (as distinct from methods) that involve simple data types (as distinct from, say, arrays or objects) are supported outside of the Access user interface; for an approximate list of function names see the 'Use Sandbox mode operations with Jet 4.0 Service Pack 3 and later' section of this MSDN article.

Also, the functions reference in the VBE help should be a starting place.

The help files are not perfect, but a little searching ought to get you what you need.

The WorksheetFunction.Floor Method in VBA is a little different from the worksheets =Floor function:

 WorksheetFunction.Floor(Arg1, Arg2)

Both arguments Arg1 and Arg2 need to be of the type double.

So you will need to use 1/24 instead of "1:00" (which is the same because 1 hour is 1/24 of a day) and a value .Cells(i, "A").Value instead of a cell reference name "A:A". Also you will need a loop to achieve it for every used cell in the whole column A.

Option Explicit 'First line at your module ensures you declare any variables

Public Sub FloorFormat()
    Dim lastRow As Long
    
    With Worksheets("Sheet1") 'Your sheet name here
        lastRow = .Range("A" & .Rows.Count).End(xlUp).Row 'find last used row
        
        Dim i As Long
        For i = 1 To lastRow 'do the following for all used rows in column A
            .Cells(i, "A").Value = Application.WorksheetFunction.Floor(.Cells(i, "A").Value, 1 / 24)
        Next i
    End With
End Sub