I've already found this in the wiki:
5.1 Excel
Date, Date-Time, and Time fields are all exported as their display values, displayed using a custom format instead of the system date format. Duration fields, however, export as the value stored in the database, which is an integer value of seconds.
The last part about duration fields in seconds is REALLY undesirable; customers are complaining that the report is useless. Is there a way to configure this to be something friendly like hours?
Thanks!
