The first iteration is more "expensive", the calendar data structures having to be built.
1 : 2 : 3 : 4 : 5 : 6 : 7 : 8 : 9 : 10 :Took: 0.0019099712371826 seconds
This second iteration is faster, the data structures being re-used
1 : 2 : 3 : 4 : 5 : 6 : 7 : 8 : 9 : 10 :Took: 0.0009310245513916 seconds