[TW-1601] Taskwarrior creates recurring tasks with wait dates way into the future

_neingeist on 2015-04-24T18:04:44Z says:_

Taskwarrior creates recurring tasks way into the future, if certain conditions are met:

```
$ task waiting balance

ID Age D P Project R Wait for Due Description
794 2d H finances R 2017-12-22 2.7 years 2015-04-25 balance expenseregister checken
```

This task was created with a wait date 2.7 years into the future. This seems to be the case because the parent task had a wait date set by setting a wait date on another child task earlier and answering "y" to the "Do you want to modify all pending recurrences of this same task? (yes/no)" question. When creating a new child task, recur.cpp then calculates (old_wait - old_due) with the values from the parent task, and uses this as the wait date for the newly created child task. This amounts , in this case, to 2.7y.

Please also see IRC discussion on 2014-04-24 for possible more detailed information: https://botbot.me/freenode/taskwarrior/2015-04-24/