We define a function that schedules a macro call to update the format of the calling range.

public static DateTime asyncFormatCaller()
    object caller = XlCall.Excel(XlCall.xlfCaller);
    if (caller is ExcelReference)
                // Set the desired selection, then apply formatting
                using (new ExcelSelectionHelper((ExcelReference)caller))
                    XlCall.Excel(XlCall.xlcFormatNumber, "h:mm:ss");
    return DateTime.Now;

Above we use the following helper class to keep track of the current selection in a macro, so that the selection is correctly restored after the macro has completed.

// Helper class to deal with Excel selections in 'using' style
public class ExcelSelectionHelper : XlCall, IDisposable
    object oldScreenUpdating;
    object oldSelectionOnActiveSheet;
    object oldActiveCellOnActiveSheet;

    object oldSelectionOnRefSheet;
    object oldActiveCellOnRefSheet;

    public ExcelSelectionHelper(ExcelReference refToSelect)
        oldScreenUpdating = Excel(xlfGetWorkspace, 40);
        Excel(xlcEcho, false);

        // Remember old selection state on the active sheet
        oldSelectionOnActiveSheet = Excel(xlfSelection);
        oldActiveCellOnActiveSheet = Excel(xlfActiveCell);

        // Switch to the sheet we want to select
        string refSheet = (string)Excel(xlSheetNm, refToSelect);
        Excel(xlcWorkbookSelect, new object[]() { refSheet });

        // record selection and active cell on the sheet we want to select
        oldSelectionOnRefSheet = Excel(xlfSelection);
        oldActiveCellOnRefSheet = Excel(xlfActiveCell);

        // make the selection
        Excel(xlcFormulaGoto, refToSelect);

    public void Dispose()
        Excel(xlcSelect, oldSelectionOnRefSheet, oldActiveCellOnRefSheet);

        string oldActiveSheet = (string)Excel(xlSheetNm, oldSelectionOnActiveSheet);
        Excel(xlcWorkbookSelect, new object[]() { oldActiveSheet });

        Excel(xlcSelect, oldSelectionOnActiveSheet, oldActiveCellOnActiveSheet);

        Excel(xlcEcho, oldScreenUpdating);

An improved function could first check whether the format of the caller needs to be updated before scheduling the macro call.