Fixed time information when exporting to spreadsheet with dates as datefields and date is in a different summer/wintertime

Fix problem if no column is selected for sorting

- For Excel sheets allow to save dates as date fields or text fields
- Use US date/time format if 12 hour clock
- Format whole day events without time
- End date of whole day events is now the last day of the event
- Fixed time difference in Excel sheets, if times are stored as date fields