Bugfix: The returned timestamp was not the NEXT one

Crontab behaves not like written in the docs.
Documentation:  int unix timestamp - next execution time will be greater than given timestamp

```
"*/1 * * * *"

Before:
  In:  Monday, 05-Aug-13 14:53:27 CEST
  Out: Monday, 05-Aug-13 14:53:27 CEST

After:
  In:  Monday, 05-Aug-13 14:53:27 CEST
  Out: Monday, 05-Aug-13 14:54:27 CEST
```

Beware: This breaks the API!