Fix rouding issues

With f44eda6258bc20eae4c1e92e5f35b7ec654d54ad examples finally show the proper output:
 ./faketime -f '+2,5y x10,0' /bin/bash -c 'date; while true; do echo $SECONDS ; sleep 1 ; done'
Fri Sep  4 21:49:30 CEST 2015
0
1
2
3
...
instead of
./faketime -f '+2,5y x10,0' /bin/bash -c 'date; while true; do echo $SECONDS ; sleep 1 ; done'
Fri Sep  4 21:51:39 CEST 2015
0
0
0
0
10
10
10
10
10
10
10
10
10
10
20
20
20
...