Would like to format conditionally using VB. I know how to use "conditional
format" on a cell in the spreadsheet, but want to do this in the code.

In particular, I have thousands of rows of "events". Content of first two
cells in each are strings representing time. The function "get_diff" returns
difference, in seconds. So cell A3 contains
"='PERSONAL.xls'!Module1.get_diff(A1,A2)"

Function get_diff(str0 As String, str1 As String, Optional dbg_flag As
Boolean = False) As Double
Dim xDiff As Double
xDiff = convert_to_sec(str1, dbg_flag) - convert_to_sec(str0, dbg_flag)
If dbg_flag Then
MsgBox "Diff is " & Format(xDiff, "#######.######")
End If
get_diff = xDiff
End Function

How do I format the cell that gets the result based on value returned from
get_diff? That is the A3 (or any other cell from which I called get_diff)
gets formatted based on the result. I can't figure out how to select the
cell in the code.
I need something like:
If xDiff 3.001 Then
.....Font.Bold = True
.....Font.ColorIndex = 3
Else
.....Font.Bold = False
.....Font.ColorIndex = 10
Endif