`thismonth` give other results then `since 2021.10.1`

Wrong results
# python3 run.py thismonth
```
Hours to do: 151.20h (21.00 days)
Hours worked: 29.56h (4.11 days)
Difference: -121.64h (-16.89 days)
```

Good results
# python3 run.py since 2021.10.01
```
Hours to do: 43.20h (6.00 days)
Hours worked: 29.56h (4.11 days)
Difference: -13.64h (-1.89 days)
```