How can I create a resuable function/formula I can use in formulas in a spreadsheet? In particular I need a very simple DateDiff formula and would like a way to encapsulate this function:

```
=(((HOUR(B3) * 60) * 60) + (MINUTE(B3) * 60) + SECOND(B3)) - (((HOUR(A3) * 60) * 60) + (MINUTE(A3) * 60) + SECOND(A3))
```

(Thanks to Phil Winstanley)