Bug in timeline when the given start_date for data cube monthly is less than a period

For data cubes monthly, there is a bug in timeline generation.

```
start=2016-12-31
end=2017-03-31
```

The timeline created represents the following values:

```
2016-12-31  ->  2016-12-31
2017-01-01  ->  2011-01-31
2017-02-01  ->  2011-02-28
2017-03-01  ->  2011-03-31
```

The first value `2016-12-31  ->  2016-12-31` should be skipped.