Oldest and newest mileage can be found by adding another Load with a Group By clause that groups the table according to the Employee ID and the Month. Within this Load you can do a min(Date) and a max(Date) to find their first and last records for that employee. This would be something like:
Min(Date) as EmpMonthMinDate,
Max(Date) as EmpMonthMaxDate,
GROUP BY [Obilasci mjesec], KOM;
You would then need to join these MinDate and MaxDate fields from the new table onto the original table:
LEFT JOIN (Obilasci) LOAD
Be sure there are no exact combinations of dates by testing how many records there are in Obilasci before and after this join. If there are more then you may have to add a key to the table or do some more groupings.
Finally, with everything on the same table, reload the table and have a test for which records are the min and max dates for an employee, with the milage being saved as a new field:
if(EmpMonthMinDate,[Obilasci stanje KM]) AS EmpMonthStartKm,
if(EmpMonthMaxDate,[Obilasci stanje KM]) AS EmpMonthEndKm
Note, you could add a further Join to calculate the difference betwee start and end for the month's mileage.