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"
```
We can block magic strings such as `10 seconds ago`, but going into this direction `DateTime` objects should be also blocked because the user can simply do `new DateTime("10 seconds ago");`.
This way it becomes very limiting because DateTime objects are the most basic type that the Generator should accept. Even if you only accept `Y-m-d` I can do `date("Y-m-d")` which is not a constant, so this issue cannot be dealt with at the input validation level.