Date generator

It is not clear to me how to shrink dates: bringing them towards 1970 (or the lower limit) does not seem to simplify much.
Another option would be to shrink in detail:
```
"2014-12-22T14:10:34"
"2014-12-22T14:10:00"
"2014-12-22T14:00:00"
"2014-12-22T00:00:00"
"2014-12-01T00:00:00"
"2014-01-01T00:00:00"
"1970-01-01T00:00:00"
```
I think we can ask for a specific time format and use `DateTime::createFromFormat` and avoid surprises.