Wrong average balance for for transactions during DST?

[John Ralls]

> 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

Regards,
John Ralls