#112 Do not crash if there is no start_date (fix #108)

Though the clean_fields() method checks the start_date field and is executed before the clean() method, the ValidationError is caught and the clean() method is called anyway. Hence, the clean() method can be called even if there is no start_date.

This prevents a crash in the clean() method even if there is no start_date.

https://code.ffdn.org/FFDN/coin/pulls/112/files#diff-96ec97a52954e15d0aa4aed2daece01e79a14e5R10 please share previous datetime module import line.

Updated to use date.today().

Thanks.

That was not exactly my point, but that looks perfectly fine also :-)