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Fix year calculation for the last day of a leap year. #1550

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Dec 31, 2020
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Fix year calculation for the last day of a leap year. #1550

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6 changes: 6 additions & 0 deletions Release/src/utilities/asyncrt_utils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -736,6 +736,12 @@ static compute_year_result compute_year(int64_t secondsSince1900)
int secondsInt = static_cast<int>(secondsLeft - year4 * SecondsIn4Years);

int year1 = secondsInt / SecondsInYear;
if (year1 == 4)
{
// this is the last day in a leap year
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This comment is a little confusing since we are talking about a day, but the variable is year.

year1 = 3;
}
Comment on lines +739 to +743
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Given this is being ported here, I want to understand it a bit better:
Azure/azure-sdk-for-cpp#1254

However, it has an edge case where the current year is the last day of a leap year, such that SecondsIn4Years contains that extra leap day but the calculation of year didn't account for the current date being exactly that extra day.

I don't fully follow what you mean here. Can you please elaborate, with a concrete example. Also, what do you mean by the "current year is the last day of a leap year"?

Lastly, what is an example impact of this bug exactly to an end user?

cc @antkmsft

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I don't fully follow what you mean here. Can you please elaborate, with a concrete example.

2020-12-31 results in year1 being 4, despite this code expecting all blocks of 4 years to be handled by SecondsIn4Years.

Also, what do you mean by the "current year is the last day of a leap year"?

Should be current "current day is the last day of a leap year"

Lastly, what is an example impact of this bug exactly to an end user?

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Lastly, what is an example impact of this bug exactly to an end user?

It incorrectly prints Thursday 2020-12-31 as Thu 2021-01-01


secondsInt -= year1 * SecondsInYear;

// shift back to 1900 base from 1601:
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15 changes: 15 additions & 0 deletions Release/tests/functional/utils/datetime.cpp
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Expand Up @@ -133,6 +133,21 @@ SUITE(datetime)
TestDateTimeRoundtrip(_XPLATSTR("9999-12-31T23:59:59Z"));
}

TEST(parsing_time_roundtrip_year_2016)
{
TestDateTimeRoundtrip(_XPLATSTR("2016-12-31T20:59:59Z"));
}

TEST(parsing_time_roundtrip_year_2020)
{
TestDateTimeRoundtrip(_XPLATSTR("2020-12-31T20:59:59Z"));
}

TEST(parsing_time_roundtrip_year_2021)
{
TestDateTimeRoundtrip(_XPLATSTR("2021-01-01T20:59:59Z"));
}

TEST(emitting_time_correct_day) {
const auto test = utility::datetime() + UINT64_C(132004507640000000); // 2019-04-22T23:52:44 is a Monday
const auto actual = test.to_string(utility::datetime::RFC_1123);
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