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Date: Sat, 28 Oct 2023 18:53:51 +0100
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To: Saki Takamachi <saki@sakiot.com>
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Subject: Re: [PHP-DEV] New DateTime[Immutable]::createFromTimestamp
From: rowan.collins@gmail.com (Rowan Tommins)

On 28/10/2023 16:47, Saki Takamachi wrote:
> Yes, I'm assuming a timestamp that includes up to microseconds. This 
> is because in the last example of the PR description, the microsecond 
> timestamp was expressed as a float.


Floating point numbers don't suddenly become unpredictable after a 
certain point, nor is there a strict relationship with any number of 
decimal digits. Rather, their absolute precision varies based on their 
magnitude - for each power of 2 in magnitude, they lose a power of 2 in 
absolute precision.

More specifically, an IEEE float64 / double precision has 53 significant 
bits, which can be allocated to either the whole or fractional part of a 
number. It has a precision of at least 1 up to 2**53-1, at least 0.5 up 
to 2**52-1, and so on - a precision of 1/2**(53-n) for values up to (2**n)-1

Plugging that formula into a spreadsheet, I get:

* accuracy to nearest microsecond up to the year 2242 (1/2**20 up to 
2**33-1 seconds)
* accuracy to nearest 10 microseconds up to the year 4147 (1/2**17 up to 
2**36-1 seconds)
* accuracy to nearest millisecond up to the year 280701 (1/2**10 up to 
2**43-1 seconds)

An application that needs to represent a timestamp more than 200 years 
in the future, with a precision of 1 microsecond, is going to be very rare.


> Another problem is that there are "millisecond" and "microsecond" timestamp formats that do not include dots, how should they be handled? In reality, such a use case is unlikely, but it is virtually impossible to tell the difference between the two


Distinguishing between the two is up to the user, just as it is up to 
the user to feed the right format to createFromFormat(). If they know 
they have a number of microseconds, they could use 
DateTimeImmutable::createFromTimestamp($value / 1_000_000) - as 
discussed above, this will not lose any accuracy for dates in the next 
200 years or so.


Regards,

-- 
Rowan Tommins
[IMSoP]