Hi Amon,
The usage of longdate seems to be faulty.
Can you try
1. Convert decimal into string(The method you are using will work for that)
2. Using the midstr() or any other string function to cut the string into parts
3. Storing the results into different columns where you have the Year, Month, Day, Hour, Minute and Seconds.( You may decide to hide these columns from output)
4. Then use these to combine into date with the function date() into one Final Date Calclated Column.
Refer the developer guide Section 5.4.5 available here http://help.sap.com/hana_appliance/ for function usage.
--Shreepad