Better date compatibility with Excel

I had issues with the ISO date format. 

My solution is to transform the date to an actual Excel date. It's the number of days since 01/01/1900:

```
 //In Excel date is a number of days since 01/01/1900
 //           timestamp in ms    to days      + number of days from 1900 to 1970
 return Number(value.getTime()/(1000*60*60*24))+25569 + 1); //+ 1 day because:
 // Excel incorrectly regards 1900 as a leap year and allows February 29 to be entered as a date in this year.
 // This is for compatibility with Lotus 1-2-3 which also had this bug
```

Sorry that my IDE removed unnecessary blank lines.