Question: How do I configure vim to open a default file (in my case, `~/Desktop/now.md`) if the `vim` command is invoked without arguments on the command-line?

Answer: 
You're asking the user for a date in the format `YYYY-MM-DD` but then trying to parse it according to this format `%d/%m/%y`.

You instead should parse the string in the same way that you requested it, [`%Y-%m-%d`](https://docs.python.org/2/library/datetime.html#strftime-and-strptime-behavior)
You're looking for format `%d/%m/%y` and asking for `%Y-%m-%d`

```
> date_visit = '2016-11-23'
> datetime.datetime.strptime(date_visit, "%Y-%m-%d")
datetime.datetime(2016, 11, 23, 0, 0)

```

Some notes:

* `%Y` : Year in four digits   
`%y` : year in two digits.
* `%d/%m/%y` translates to *"day of month in one or two digits, `/`, month of year in one or two digits, `/` year in two digits".*
* `%Y-%m-%d` translates to *"four-digit-year, `-`, month-of-year, `-` day-of-month"*