The main problem with this is that there are so many different import formats for dates and times and this is just one of many. :( Not sure yet.

I think maybe we do not need to cover so many different formats. But just some common format, like 2014-04-30 16:22:25, 20140430162225 or 2014.04.30 etc.

but what about 24-12-2014?

How about use common format in default, but if you want a custom format of date. you can do this in your code:

@click.option('--start', '-s', type=click.DateParamType('%d-%m-%Y'))

or just like this:
@click.option('--start', '-s', type=click.DATE('%d-%m-%Y'))

I'm not convinced yet that this is a good idea. Try to convince me :)

Closing it for the time being.