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)
Your prompt asks you to enter in `YYYY-MM-DD`, but your `strptime` is attempting to use the format `%d/%m/%y`. You need to have the formats to match for `strptime` to work

```
import datetime

d = '2016-11-21'
d = datetime.datetime.strptime(d, "%Y-%m-%d")
d = datetime.datetime(2016, 11, 21, 0, 0)

>>> 2016-11-21 00:00:00

d = '11/21/2016'
d = datetime.datetime.strptime(d, "%d/%M/%Y")
d = datetime.datetime(2016, 11, 21, 0, 0)

>>> 2016-11-21 00:00:00

```

I personally like to use the `python-dateutil` module for parsing date strings that allows for different formats

> 
> pip install python-dateutil
> 
> 
> 

```
from dateutil.parser import parse

d1 = 'Tuesday, October 21 2003, 12:14 CDT'
d2 = 'Dec. 23rd of 2012 at 12:34pm'
d3 = 'March 4th, 2016'
d4 = '2015-12-09'

print(parse(d1))
>>> 2003-10-21 12:14:00

print(parse(d2))
>>> 2012-12-23 12:34:00

print(parse(d3))
>>> 2016-03-04 00:00:00

print(parse(d4))
>>> 2015-12-09 00:00:00

```