Fixed monthly cycling - again!

This was still wrong (in a way that was not tested by the new Monthly cycling test). The DDHHmmss part of the cycle time was being taken from the start T instead of the anchor T, in the initial adjustment to find the cycle time of the first task instance.  E.g. this sequence:

```
Monthly( 201001,3)
```
for an initial cycle time of 20130128 should yield the sequence 2013040100, 2013070100, ...; whereas on current master it yields 2013012800, 2013042800, ...

@arjclark - please review.