Simple Visual Basic for Applications project. The VBScipt will be a macro attached to a Microsoft Excel document. Upon opening the .xlsm file, there is a single reminder event attached and the public at_time() procedure is called. The at_time() procedure is also recursive and calls itself to persist the attached reminder event as long as the file is open. A loop is used to look through the first 1000 rows of two different sheets. The script then compares today's date (and today's date plus 3 days) with the date values provided in the Excel sheets. When there is a match, a message box pop up is generated to inform the user about the reminder.
For more information you can look at Microsoft's Visual Basic documentation.