dayofyear is not great when going into a new year

[Michael F. Stemper]

On 05/01/2021 15.27, Chris Angelico wrote:
> On Wed, Jan 6, 2021 at 8:02 AM Martin Schöön <martin.schoon at gmail.com> wrote:

>> 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.

Possibly better than rolling your own is using standard stuff:

 >>> from datetime import date,datetime
 >>> datetime.toordinal(date(2020,1,1))
737425
 >>> datetime.toordinal(date(2021,1,5))
737795
 >>> datetime.toordinal(date(2021,1,5)) - datetime.toordinal(date(2020,1,1))
370
 >>>

-- 
Michael F. Stemper
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