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Posted by Rik on 08/15/07 10:54
On Wed, 15 Aug 2007 12:41:05 +0200, gosha bine <stereofrog@gmail.com> =
wrote:
> On 15.08.2007 12:26 Rik wrote:
>> On Wed, 15 Aug 2007 12:23:52 +0200, gosha bine <stereofrog@gmail.com>=
=
>> wrote:
>>
>>> On 15.08.2007 11:57 FFMG wrote:
>>>> Hi, Given a time I want to know if it is today or yesterday, (on t=
he
>>>> server).
>>>> I cannot use time difference because if I log the time and it is =
>>>> 23:59
>>>> and I check again at 24:01 then the earlier time is, in fact,
>>>> yesterday.
>>>> so given time $a and time $b how can I calculate if $a is the day
>>>> before $b?
>>>>
>>>
>>> if(unixtojd($b) - unixtojd($a) =3D=3D 1)
>>>
>>> http://www.php.net/manual/en/function.unixtojd.php
>> Erm, Julian days start at noon?
>
> Don't trust every comment you see in the manual. ;)
Haven't examined the function, but I did see that comment yeah, and =
wikipedia (well, another source of blatant lies sometimes, but hey) stat=
es:
<http://en.wikipedia.org/wiki/Julian_day>
[quote]
The Julian day or Julian day number (JDN) is the integer number of days =
=
that have elapsed since the initial epoch defined as noon Universal Time=
=
(UT) Monday, January 1, 4713 BC in the proleptic Julian calendar [1]. Th=
at =
noon-to-noon day is counted as Julian day 0. Thus the multiples of 7 are=
=
Mondays. Negative values can also be used, although those predate all =
recorded history.
Now at 00:20, Friday August 10, 2007 (UTC) the JDN is 2454322. The =
remainder of this value divided by 7 is 3, an integer expression for the=
=
day of the week with 0 representing Monday.
The Julian date (JD) is a continuous count of days and fractions elapsed=
=
since the same initial epoch. Currently the JD is 2454322.51389. The =
integral part (its floor) gives the Julian day number. The fractional pa=
rt =
gives the time of day since noon UT as a decimal fraction of one day or =
=
fractional day, with 0.5 representing midnight UT. Typically, a 64-bit =
floating point (double precision) variable can represent an epoch =
expressed as a Julian date to about 1 millisecond precision.
[/quote]
Which led me to assume it to be true.
Then again, the manual points at =
<http://www.hermetic.ch/cal_stud/jdn.htm>, which states there are severa=
l =
posibilities to start the day... If you say the PHP function takes =
midnight, I believe you :-)
-- =
Rik Wasmus
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