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

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

and so on.

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