Wrong average balance for for transactions during DST?

[Mosè Giordano]

Hi John,

2015-04-23 18:39 GMT+02:00 John Ralls <jralls at ceridwen.us>:
>
>> On Apr 22, 2015, at 2:14 PM, Mosè Giordano <mose at gnu.org> wrote:
>>
>> Hi all,
>>
>> consider the attached account.  If I open the yearly average balance
>> report (Reports > Assets & Liabilities > Average Balance, with Year as
>> Step size) I get an average balance of 254.04$, instead of the
>> expected integer 254$, why?  I noticed the extra decimals appear if
>> the transaction is dated during daylight saving time, and disappear
>> during standard time.  Is this an intended, yet unexpected, behavior?
>
> I think that this is an expected, yet unintended behavior caused by the way the report calculates the average and the way GnuCash handles transaction posted dates.
>
> GnuCash converts the posted date to a timestamp of midnight local time: The transaction in your test file has a date-posted of 2015-04-22 0:00:00 +02:00. This is represented in GnuCash's binary as a 64-bit number of seconds from 1970-01-01 0:00:00 UTC, which we call a time64. Without looking at the code in the report, I'll speculate that it's calculating the beginning and end of the year as time64s as well. If we have T_trans, T_begin, and T_end representing those time64s for the transaction's date-posted, beginning of the year, and end of the year, I expect that the averaging calculation looks like
>
>      T_amount * (T_end - T_trans) / (T_end - T_begin)
>
> Since DST times look like they're occurring at 01:00 instead of 00:00 the average gets shifted. I'm able to replicate your shift as follows:
>   The date entered, 4/22, is the 111th day of the year, so there are 254 days remaining.
>   (365 * (254 * 24 + 1)) / (365 * 24) = 254.0417

Thanks for the explanation. Is there a way to get the correct average
balance?  It would be great to automatically have the right figure,
and I think everyone using this report expects the same.  Is this
worth filing a bug report?

Bye,
Mosè