Analytics Software

hi all

when i was trying to perform Time series functions ..why we have to select the chronological key..?
i have created time dimension..and i have created year,month,week,day...levels in year level i have added calendar_yaer and
calendar_year_id,....and i defined two columns as key values.... and calendar_year for drill down...
in that if i use calendar_year as chronological key what would be the result...
if i use caleendar_year_id as chronological key what would be the result....
i tried it and both are giving same result...

i don't under stand why we have to select the chronological key???

please any one tel me about it

Thanks