The function VBA.Format should be part of the Excel object model and not only the VBA language
The Format (or VBA.Format) function is part of the VBA language, not the Excel object model. This means that this function is not available to .NET COM addins (VSTO, AddinExpress, Excel DNA). This can be a problem for the development of new addins using .NET or upgrading existing VBA addins to COM.
Note that .NET has string.Format() and Microsoft.VisualBasic.Strings.Format() but neither of those has the same behavior as the VBA Format function especially with regards to dates.
Note that there is a similar function that is part of the Excel object model: WorksheetFunction.Text(). However, this function still differs from the VBA Format() function. It would be nice if the Format function could also be included in the excel object model as WorksheetFunction.Format() so that it can be accessed from .NET addins