"this x" is not recognized

GNU date recognizes "this x", for example:

```
$ date --date="this year"
Tue Jun 20 10:50:27 AM CEST 2023
$ date --date="this week"
Tue Jun 20 10:50:51 AM CEST 2023
```
uutils date, using `parse_datetime`, doesn't recognize "this x" and returns an "invalid date" error instead.