dayofyear is not great when going into a new year

[Chris Angelico]

On Wed, Jan 6, 2021 at 8:02 AM Martin Schöön <martin.schoon at gmail.com> wrote:
>
> Hello,
>
> I have had some Python fun with COVID-19 data. I have done
> some curve fitting and to make that easier I have transformed
> date to day of year. Come end of 2020 and beginning of 2021
> and this idea falls on its face.
>

There are multiple definitions for "day of year", depending on how you
want to handle certain oddities. The simplest is to identify Jan 1st
as 1, Jan 2nd as 2, etc, to Dec 31st as either 365 or 366; but some
libraries will define the year as starting with the week that contains
the Thursday, or something, and then will define days of year
accordingly.

If you want an easy way to graph day-by-day data and the exact day
numbers are irrelevant, what I'd recommend is: Convert the date into
Unix time, divide by 86400, floor it. That'll give you a Julian-style
date number where Jan 1st 1970 is 0, Jan 2nd is 1, etc, and at the end
of a year, it'll just keep on incrementing. That would get you past
the 2020/2021 boundary pretty smoothly.

ChrisA