'1 year' = '360 days' ????

[Ricardo Perez Lopez]

Hello everyone:

I'm a PostgreSQL newbie, working now with dates, times, timestamps and
intervals.

I have three questions about the above:

FIRST:
--------

I have observed that, for PostgreSQL, one year is actually 360 days:

SELECT '1 year'::timestam p = '360 days'::timestam p;

?column?
-------------
t
Glubs! I believed that 1 year is 365 days, or 366 if leap. Is it normal?
SECOND:
-----------

When I want to check how many time is between two dates, I have two options
(which shows two different results):

SELECT '30-09-04'::timestamp - '30-09-03'::timestamp,
age('30-09-04'::timestamp, '30-09-03'::timestamp) ;

?column? | age
-------------------------------
@ 366 days | @ 1 year
The results are different. If we compare the two results:

SELECT ('30-09-04'::timestamp - '30-09-03'::timestamp) =
age('30-09-04'::timestamp, '30-09-03'::timestamp) ;
?column?
--------------
f
Obviously, it returns False, because I told in the first question, 1 year is
360 days for PostgreSQL.

The question is: is it normal? Which of the two methods is the correct? To
substract timestamps? Or to use the age function?
THIRD:
--------

As I told in the second question, when I do:

SELECT '30-09-04'::timestamp - '30-09-03'::timestamp;

the result is:

?column?
--------------
@ 366 days

The question is: is there any way to "normalize" the result, such that the
result was:

@ 1 year 1 day

?

I think it's better (and more correct) "@ 1 year 1 day" rather than "@ 366
days". Is there any way to achieve that?

Thanks to all.

Ricardo.

_______________ _______________ _______________ _______________ _____
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[Pierre-Frédéric Caillaud]

problem is that '1 months':: interval does not have the same value if you
add it to a date or another :

=> SELECT '2004-02-01'::timestamp+ '1 month'::interva l,
'2004-03-01'::timestamp+ '1 month'::interva l;
?column? | ?column?
---------------------+---------------------
2004-03-01 00:00:00 | 2004-04-01 00:00:00

SELECT '2004-03-01'::timestamp-'2004-02-01'::timestamp,
'2004-04-01'::timestamp-'2004-03-01'::timestamp;
?column? | ?column?
----------+----------
29 days | 31 days

That's because a month is an undefined number of days (also some years
are 366 days). In that case '1 months':: interval is either 29 or 31 days
but it could be 28 in february 2003 or 30 in april !

Thus if we have a date d and two intervals i1 and i2 :

The comparison (d+i1) < (d+i2) depends on the value of d (and the
timezone).
For instance if i1 is '1 month' and i2 is '30 days', we have :

SELECT '2004-02-01'::timestamp+ '1 month'::interva l,
'2004-02-01'::timestamp+ '30 days'::interval ;
?column? | ?column?
---------------------+---------------------
2004-03-01 00:00:00 | 2004-03-02 00:00:00

Thus (d+i1) < (d+i2)
SELECT '2004-04-01'::timestamp+ '1 month'::interva l,
'2004-04-01'::timestamp+ '30 days'::interval ;
?column? | ?column?
---------------------+---------------------
2004-05-01 00:00:00 | 2004-05-01 00:00:00

Thus (d+i1) = (d+i2)

SELECT '2004-03-01'::timestamp+ '1 month'::interva l,
'2004-03-01'::timestamp+ '30 days'::interval ;
?column? | ?column?
---------------------+---------------------
2004-04-01 00:00:00 | 2004-03-31 00:00:00

Thus (d+i1) > (d+i2)

And that's normal ! Intervals having months are extremely useful to
express the idea of 'same day, next month' that you can't do with just an
interval expressed in seconds. However, beware :

SELECT '2004-01-31'::timestamp+ '1 month'::interva l;
?column?
---------------------
2004-02-29 00:00:00
(1 ligne)

SELECT '2004-01-30'::timestamp+ '1 month'::interva l;
?column?
---------------------
2004-02-29 00:00:00
(1 ligne)

SELECT '2004-01-29'::timestamp+ '1 month'::interva l;
?column?
---------------------
2004-02-29 00:00:00
(1 ligne)

SELECT '2004-01-28'::timestamp+ '1 month'::interva l;
?column?
---------------------
2004-02-28 00:00:00
31 january + 1 month = 29 february (it clips at the end of the month,
which is IMHO GOOD).

How can we sort intervals meaningfully in these conditions ? Can we ? In
fact the value of an interval depends on the application, and intervals
with months are in another 'world' than intervals with only seconds...
same thing for years.

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[Michael Glaesemann]

On Oct 24, 2004, at 4:13 PM, Pierre-Frédéric Caillaud wrote:

How can we sort intervals meaningfully in these conditions ? Can we ?
In fact the value of an interval depends on the application, and
intervals with months are in another 'world' than intervals with only
seconds... same thing for years.

Added to this, I've been wondering whether '1 day'::interval is also
problematic wrt daylight savings time or changing time zones. The whole
thing seems pretty hairy to me.

Michael Glaesemann
grzm myrealbox com
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[Tom Lane]

Michael Glaesemann <gr**@myrealbox .com> writes:

Added to this, I've been wondering whether '1 day'::interval is also
problematic wrt daylight savings time or changing time zones.

This is exactly the point I alluded to earlier: intervals need to have
three components (months, days, seconds) not just two. That's been on
the to-do list for quite awhile. All the other units we support for
intervals bear a fixed relationship to one or another of these, so
three is sufficient.

Question to think about: should we allow fractional months or days in
the stored representation? There are some places where the existing
restriction that the months field is an integer requires awkward
compromises. On the other hand, it's not real clear what a fractional
month actually means, and similarly a fractional day is hard to assign
meaning to without positing that 1 day == 24 hours.

regards, tom lane

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[Bruno Wolff III]

On Sun, Oct 24, 2004 at 11:29:13 -0400,
Tom Lane <tg*@sss.pgh.pa .us> wrote:

Question to think about: should we allow fractional months or days in
the stored representation? There are some places where the existing
restriction that the months field is an integer requires awkward
compromises. On the other hand, it's not real clear what a fractional
month actually means, and similarly a fractional day is hard to assign
meaning to without positing that 1 day == 24 hours.

There are reasonable addition and subtraction operation definitions
on two intervals. There might be some application where you want to
keep track of fractional months or days. What I am not sure of is
would you really have a reason to add fractional months or days
to a timestamp. There are a couple reasonable definitions you might
make for this definition, but I don't really see a good reason to
want this. ne thing to note, when adding intervals you can add the
fractions normally. When adding to a date you can get the actual length
of the day or month the fractional part adds to, if you wanted to use
that information. (Though the resulting day or month may not be the
one you added the fractional month to.)

I think starting with a type where months and days were integers would be
OK, since you could generalize it to handle fractional months and days
later and not break applications.

Another thing to think about when designing this type, is that when
adding timestamps and intervals it makes a difference in which order
you add the months, days and seconds.

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[Tom Lane]

Bruno Wolff III <br***@wolff.to > writes:

Wikipedia gives 365.242189670 days (86400 seconds) as the length of
the mean solar year in 2000. To give you some idea of how constant
that values is, Wikipedia claims that 2000 years ago the mean solar
year was about 10 seconds longer. Using the above value I get there
is an average of 2629743 seconds in a month. And yet another option is to note that in the Gregorian calendar there are
400*365+97 days or 400*12 months in 400 years, which gives 2629746 seconds
per month on average.

I like the latter approach, mainly because it gives a defensible
rationale for using a particular exact value. With the solar-year
approach there's no strong reason why you should use 2000 (or any other
particular year) as the reference; and any value you did use would be
subject to both roundoff and observational error. With the Gregorian
calendar as reference, 2629746 seconds is the *exact* answer, and it's
correct because the Pope says so ;-).

(Or, for the Protestants among us, it's correct because the SQL standard
specifies use of the Gregorian calendar.)

regards, tom lane

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[Guy Fraser]

Tom Lane wrote:

Doug McNaught <do**@mcnaught. org> writes:

template1=# select '1 year'::interval = '360 days'::interval ;
?column?
----------
t
(1 row)

Yeah, if you look at interval_cmp_in ternal() it's fairly obvious why.
I think that this definition is probably bogus, and that only intervals
that match exactly (equal months parts *and* equal seconds parts) should
be considered "equal". However the most obvious way to redefine it
(compare the months, and only if equal compare the seconds) would lead
to rather nonintuitive behaviors such as "'1 year' > '1000 days'".
Anybody have any thoughts about a better way to map the multicomponent
reality into a one-dimensional sorting order?

(Note also that as Bruno was just mentioning, we really ought to have
months/days/seconds components, not just months/seconds; which makes the
comparison issue even more interesting.)

regards, tom lane

As any of us who have ever researched how to calculate time know;

1) The amount of time in 1 year depends on the year due to
leap years.
2) The amount of time in 1 month depends on the month and year
because a month is an arbitrary number of days.
3) A week is a theological creation always equal to 7 days.

Using the Gregorian Calendar there are 10 missing days between
Oct. 4, 1582 and Oct. 15, 1582 . Leap Years are (((every 4 years)
except when modulo 100) except when modulo 400).

It is therefore not possible to define a Month or Year in Seconds,
without knowing which Day, Month and Year you calculating.

Time constants :

1 Solar Day = 23 hours 56 minutes 4.091 seconds
1 Lunar Month = 27.32158 days
1 Tropical Year = 365.24215 Solar Days

1 Year in Gregorian time is :
365 Days 5 Hours 49 Minutes 12 Seconds

As it is now obvious there is not any simple way to convert
months to seconds since a month is an abstract number of days used
to split four (13 week) seasons three ways plus one day every non
leap year and two days every leap year.

When calculating any usage based on time, it is a good idea to
store usage in days:hours:minu tes:seconds because they are static
and stable, if you discount the deceleration of the earth and
corrections in leap seconds for atomic clocks [see
http://tycho.usno.navy.mil/leapsec.html ].

Trivia: In approximately 620 million years a day will be twice as
long as it is today.

--
Guy Fraser
Network Administrator
The Internet Centre
780-450-6787 , 1-888-450-6787

There is a fine line between genius and lunacy, fear not, walk the
line with pride. Not all things will end up as you wanted, but you
will certainly discover things the meek and timid will miss out on.


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[Guy Fraser]

Tom Lane wrote:

Bruno Wolff III <br***@wolff.to > writes:

Wikipedia gives 365.242189670 days (86400 seconds) as the length of
the mean solar year in 2000. To give you some idea of how constant
that values is, Wikipedia claims that 2000 years ago the mean solar
year was about 10 seconds longer. Using the above value I get there
is an average of 2629743 seconds in a month.
And yet another option is to note that in the Gregorian calendar there are
400*365+97 days or 400*12 months in 400 years, which gives 2629746 seconds
per month on average.

I like the latter approach, mainly because it gives a defensible
rationale for using a particular exact value. With the solar-year
approach there's no strong reason why you should use 2000 (or any other
particular year) as the reference; and any value you did use would be
subject to both roundoff and observational error. With the Gregorian
calendar as reference, 2629746 seconds is the *exact* answer, and it's
correct because the Pope says so ;-).

(Or, for the Protestants among us, it's correct because the SQL standard
specifies use of the Gregorian calendar.)

regards, tom lane

Give or take one day every 4000 years. ;-)

--
Guy Fraser
Network Administrator
The Internet Centre
780-450-6787 , 1-888-450-6787

There is a fine line between genius and lunacy, fear not, walk the
line with pride. Not all things will end up as you wanted, but you
will certainly discover things the meek and timid will miss out on.


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[Gaetano Mendola]

Guy Fraser wrote:

Trivia: In approximately 620 million years a day will be twice as long
as it is today.

Do you think then that Postgres628M.0 will fix it ? :-)

Regards
Gaetano Mendola

[Guy Fraser]

Gaetano Mendola wrote:

Guy Fraser wrote:

Trivia: In approximately 620 million years a day will be twice as
long as it is today.

Do you think then that Postgres628M.0 will fix it ? :-)

Regards
Gaetano Mendola

I just hope, I don't have to work an equivalent fraction of the day for the
same pay, but with any luck I'll have all my bills paid and be retired by
then. ;-)

--
Guy Fraser
Network Administrator
The Internet Centre
780-450-6787 , 1-888-450-6787

There is a fine line between genius and lunacy, fear not, walk the
line with pride. Not all things will end up as you wanted, but you
will certainly discover things the meek and timid will miss out on.


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[Bruno Wolff III]

On Wed, Oct 27, 2004 at 16:26:13 -0600,
Guy Fraser <gu*@incentre.n et> wrote:

When calculating any usage based on time, it is a good idea to
store usage in days:hours:minu tes:seconds because they are static
and stable, if you discount the deceleration of the earth and
corrections in leap seconds for atomic clocks [see
http://tycho.usno.navy.mil/leapsec.html ].

The length of calendar days isn't constant. In many timezones, one day a year
is 23 hours long and another is 25 hours long.

Having month and year intervals is useful for events that repeat monthly or
yearly in spite of there not being a constant number of seconds between
events.

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