PostgreSQL age() function: different/unexpected results when landing in dfferent month

Question

Today, I've encountered an unexplainable result in PostgreSQL 9.6 while running this query:

SELECT age('2018-06-30','2018-05-19') AS one,
       age('2018-07-01','2018-05-20') AS two; 

Expected results for both columns: 1 mon 11 days. However, only for the interval from 2018-05-19 to 2018-06-30, I get what I expect, while for 2018-05-20 till 2018-07-01 I'll get one day more: 1 mon 12 days

I don't get why this is the case and in my understanding, between 2018-05-20 2018-07-01 is just an interval of 1 mon 11 days and the Postgres result here is wrong.

I cannot find any in-depth information on how exactly the PostgreSQL-age(timestamp,timestamp) function works. However, I assumed that function does something like: Go from the start date in month steps forward till you reach the end month. From there, go to the day of the end date. Sum up months and days.

So, in my understanding, this is what should go on under the hood in my case (sorry, for being so verbose here, but I feel it's necessary):

Start at 2018-05-19. Go one month forward. Land at 2018-06-19. Walk N days forward till you've reached 2018-06-30:

1 day: 20
2 days: 21
3 days: 22
4 days: 23
5 days: 24
6 days: 25
7 days: 26
8 days: 27
9 days: 28
10 days: 29
11 days: 30

= 1 month 11 days.

For the time between 2018-05-20 and 2018-07-01 it should be almost the same:

Start at 2018-05-20. Go one month forward. Land at 2018-06-20. Walk N days forward till you've reached 2018-07-01:

1 day: 21
2 days: 22
3 days: 23
4 days: 24
5 days: 25
6 days: 26
7 days: 27
8 days: 28
9 days: 29
10 days: 30
11 days: 1

= 1 month 11 days.

Is this my mistake or one of PostgreSQL? Are there alternative functions/algorithms which work the way I described/expect?

Answer 1

The above unexpected behaviour is not because of age() . But because of interval data type which will allows calculations. Below link contains the necessary explanation.

Odd month arithmetic

In your first one since two times are successive you don't see unexpected. But it second it is not. This tends to above odd month arithmetic behaviour

Answer 2

For those interested: I think I've found a workaround for the problem, using a function which gives me the desired result. It works according to my own tests, even for leap years, but unfortunately, I cannot guarantee that it will always work. It also seems a little bit hacky.

CREATE OR REPLACE FUNCTION age_forward ("endDate" date,"startDate" date) 
     RETURNS interval AS $$

     /*

     Basic approach: actually do a culculation like this:
     SELECT age('2018-07-01','2018-06-01') + ((30 - 20) + 1||' days')::interval;

     So, basically:
     (1) truncate start and end to month level, so always FIRST of month
     (2) add one month to the start month
     (3) calculate the days
     (4) add the days as string and build the interval

     The crucial part is 3: calculate the days

     We do it like this:
     - get the number of days for the month in question. The month in question is the month BEFORE the end month. For our example it is JUNE
     - subtract the start date day number from the number of days (here 20)
     - add the end date day number (here 1)

     */

     SELECT CASE 

        /* First step: Check if the startDate day number is lower or equal the endDate day number.
           If this is the case: Do vanilla age(). Works perfectly here
        */

        WHEN (date_part('day', "startDate" )::integer) <= date_part('day', "endDate" )::integer

        THEN age("endDate","startDate")

        /* Special case to treat here: startDate day number is greater than endDate day number. Do the algorithm described above */

        ELSE  age
               (

                  date_trunc('month', "endDate"::date), /* Go just till month level, always using '1' as day */

                  date_trunc('month', "startDate"::date)
                  + '1 mons'::interval
                  /* Add one month so that interval to look for will become actually shorter for now. */
                ) 
              + 
                (
                   (

                     /* Calculate the last day of the month previous to the end month. See https://stackoverflow.com/questions/28186014/how-to-get-the-last-day-of-month-in-postgres  */
                     (date_part('day',(date_trunc('month', (date_trunc('month', "endDate"::date) - '1 mons'::interval)  ) + interval '1 month' - interval '1 day')::date))::integer

                     - 

                     /* endDate day number subtracted */
                     date_part('day', "startDate" )::integer
                   )

                   /* endDate day number added */
                   + date_part('day', "endDate" )::integer||' days'

                )::interval
        END

$$ LANGUAGE sql;

Answer 3

This isn’t a complete answer, but it’s much too long for a comment.

The problem with the interval mon is that it’s a moving target.

For example, if you run this:

SELECT
    '2018-01-30'::date+interval'1 month' AS toomuch,    --  02-28
    '2018-01-19'::date+interval'1 month' AS jan,        --  02-19
    '2018-02-19'::date+interval'1 month' AS feb,        --  03-19
    '2018-03-19'::date+interval'1 month' AS mar,        --  … and so on…
    '2018-04-19'::date+interval'1 month' AS apr,
    '2018-05-19'::date+interval'1 month' AS may,
    '2018-06-19'::date+interval'1 month' AS jun
;   

you will get the 19th every time (except for the first, of course). That means that the mon interval has obviously changed.

Looking at the first case, then toomuch column, you can also see that it’s not just a case of adding to the month number, since the result needs to be cropped to the end of the month.

So, if you run an extended version of the original question:

SELECT
    age('2018-06-30','2018-05-19') AS one,
    '2018-06-30'::date-'2018-05-19'::date AS oneminus,
    age('2018-07-01','2018-05-20') AS two,
    '2018-07-01'::date-'2018-05-20'::date AS twominus
;

you can see that though the difference is 42 days in both cases, the 1 mon part appears to use two different versions of a month.

In other words, the interval is really the same, as long as you choose different definitions of a month.

I haven’t quite worked out how it came to use a 31-day month in the first instance and a 30-day month in the second, but it clearly has something to do with the starting or finishing date.