CODEWITHSUNDEEP
pythonpython-3.xpandasdatetime
Linus
Linus

Reputation: 125

Does Pandas account for leap years when calculating dates

I am trying to add 148.328971 years precisely from the day 01.01.2000 using pandas. I first converted this to days by multiplying it by 365. So here is my question, albeit probably a dumb one.

Does pandas consider leap years when calculating days? The obvious answer is yes because it is calculating days but I have to make sure, precision of dates and time is important in the analysis I am trying to do.

I get this result when calculating with pandas which I am not sure is entirely correct:

03.25.2148 1:47:09 AM

code being used:

import pandas as pd
start = "01/01/2000"
end = pd.to_datetime(start) + pd.DateOffset(days=54140.074415)
print(end)

Any help would be greatly appreciated! Sorry in advance if this seems to be basic knowledge but I have to be certain

Upvotes: 1

Views: 1440

Answers (2)

Mark Reed
Mark Reed

Reputation: 95315

Your question is flawed, as stated; a year is not a fixed length of time. So what does "precisely 148.328971 years" even mean? Do you mean 148 calendar years plus 0.328971 of way through the next calendar year? And if you're counting down to a precision of 0.000001 year – a millionth of a year, or about 30 seconds – then "01.01.2000" is a pretty imprecise starting point; from what time on January 1st, 2000 are you counting?

Let's assume you mean civil calendar years from midnight UTC. Then 148.0 years would get you to January 1, 2148, still at midnight UTC. Since 2148 is a leap year, 0.328971 of the way through it would add 0.328971 × 366 = 120.403 more days, which gets you to April 30, 2148 at 09:40 UTC.

Maybe you mean to be counting some "year" value that really is fixed? We do that in other contexts; the light year is based on the mean Julian calendar year, and so is defined as the distance light travels in exactly 365.25 atomic days. If you mean 148.328971 of those years, that'd be 54,177.1567 days, which would get you to May 1, 2148 at 03:45 UTC instead.

But we don't use the Julian calendar anymore for civil purposes. Maybe you want instead the mean year of the Gregorian calendar, which replaced it in the West? That's exactly 365.2425 days; 148.328971 of those years is 54,176.0442 days, from 2000 Jan 1 is back to 2148 April 30, only now at 01:03 UTC.

Then again, In parts of the world where the Eastern Orthodox Church is dominant, they instead use the Revised Julian calendar, whose mean year is 365.242̅ days (exactly 365 days, 5 hours, 48 minutes, 48 seconds). 148.328971 of those years is only 54,176.0030 days, which still get you to April 30th, but just barely over the line from the 29th at less than 5 minutes after midnight: 00:04 UTC.

So if you're counting calendar years of some description, you wind up somewhere on April 30th or May 1st, 2148. I trust this is helpful.

But maybe you mean to toss out calendars and go directly to the value they're trying to approximate: the mean tropical year! But then we have to ask where in the year you're measuring from, because December solstice to December solstice is a different length on average than March equinox to March equinox (because the length of the year itself is constantly changing). When we need a fixed value we tend to use the average of averages, as it were, taking the mean of the length values across the whole year. As of 2000 that value was about 365.24219 days. 148.328971 of those is only 54,175.9982 days, which leaves you even earlier: April 29th, 2148 at 23:57 UTC.

Then there's the sidereal year, but even though it arguably has the greatest claim at being the "real" period of the Earth's orbit, it doesn't see much use outside astronomy; probably not that.

Anyway, the real question is - where does this "148.328971" figure come from, and what is the intent behind it? Once you know what the desired answer actually is, it will be easy enough to find its value.

Upvotes: 4

Mohammad
Mohammad

Reputation: 3396

Yes, it does. However, your conversion from years to days is already ignoring the leap years. You can multiply by 365.25 (365.242, as suggested in the comments) which gives better results.

You can check the accuracy of the results on wolfram alpha: https://www.wolframalpha.com/input/?i=148.328971+years++from+01%2F01%2F2000

In addition, you can use pandas DateOffset with years. However, currently only integer values are supported.

import pandas as pd
start = "01/01/2000"
end = pd.to_datetime(start) + pd.DateOffset(years =148, days =0.328971*365.242)
print(end)

# 2148-04-30 03:45:35.229600

It seems to work well but misses by few hours.

Upvotes: 2

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