This page covers some basic knowledge necessary any SheetKraft implementation
Most windows systems are configured to hide file name extensions by default. This can cause confusion in identifying files properly.
In newer versions of Windows, this setting is controlled by the View > File name extensions check box on the ribbon in Windows Explorer. The check box should be checked.
In older versions, this setting is available from the Tools menu > Folder Options > Hide extensions for known file types. The menu can be made visible by pressing the Alt key. This checkbox should be unchecked.
The extension in a file name (example .xls or xlsx) is supposed to indicate the file format. It is used by the operating system to choose the default program to be used to open the file. Beyond this convention, there is no real connection between the file extension and the file format. Merely changing the extension has no effect on the file format or file contents. A comma separated text file can be saved with a .xls extension. The file format is comma separated values but the extension is .xls. Excel will open such a file with a warning but other programs (including SheetKraft) will not accept such a file if an Excel file format was expected.
Sometimes input files may be in a format that does not match their extension. Excel opens such files with a warning. To determine the actual type of these files, use the Save As functionality in Excel. The default selection of the format will indicate the actual format of the file regardless of the incorrect extension.
The links below contain introductory material that covers this topic.
Basic Excel functions (SUM, IF, COUNT, MAX, MIN, DATE, YEAR, MONTH, DAY, IFERROR, IFNA, INDEX, TODAY, EDATE, YEARFRAC, XIRR, XNPV formulas)