Custom Excel Function Toolkit

I need a clean, easy-to-maintain Excel workbook that relies heavily on built-in functions so I can run everyday calculations without manual effort. The file should demonstrate and automate:

• Mathematical & statistical tasks (e.g., dynamic SUM, AVERAGE, COUNTIF scenarios).
• Text & string processing (such as CONCAT, LEFT/RIGHT, and quick data-cleanup formulas).
• Date & time handling (rolling deadlines with TODAY/NOW, elapsed-time checks with DATEDIF, etc.).

Please structure the sheets logically, label inputs and results clearly, and add brief in-cell comments so I can understand or tweak each formula later. A short read-me tab outlining what goes where and how to extend the logic will be appreciated. No macros—just native Excel functions that a typical user can audit and repurpose.