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averyk22 merged 1 commit into from
Sep 24, 2026
+69 −1
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Preserving millisecond precision in DateTime parser #3934

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6 changes: 6 additions & 0 deletions .changelog/bugfix-datetime-milliseconds.json
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
{
"type": "bugfix",
"category": "aws-cpp-sdk-core",
"contributor": "averyk22",
"description": "Preserve millisecond precision when constructing DateTime from an ISO-8601 string."
}
18 changes: 17 additions & 1 deletion src/aws-cpp-sdk-core/source/utils/DateTimeCommon.cpp
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Expand Up @@ -384,7 +384,7 @@ namespace {
class DateParser
{
public:
DateParser(const char* toParse) : m_error(false), m_toParse(toParse), m_utcAssumed(true)
DateParser(const char* toParse) : m_error(false), m_toParse(toParse), m_utcAssumed(true), m_milliseconds(0)
{
m_parsedTimestamp = CreateZeroedTm();
memset(m_tz, 0, 7);
Expand All @@ -395,6 +395,7 @@ class DateParser
virtual void Parse() = 0;
bool WasParseSuccessful() const { return !m_error; }
std::tm& GetParsedTimestamp() { return m_parsedTimestamp; }
int GetParsedMilliseconds() const { return m_milliseconds; }
bool ShouldIAssumeThisIsUTC() const { return m_utcAssumed; }
const char* GetParsedTimezone() const { return m_tz; }

Expand All @@ -405,6 +406,7 @@ class DateParser
bool m_utcAssumed;
// The size should be at least one byte greater than the maximum possible size so that we could use the last char to indicate the end of the string.
char m_tz[7];
int m_milliseconds{0};
};

static const int MAX_LEN = 100;
Expand Down Expand Up @@ -852,6 +854,10 @@ class ISO_8601DateParser : public DateParser
m_state = 7;
stateStartIndex = index + 1;
}
else if (isdigit(c) && (index - stateStartIndex) < 3)
{
m_milliseconds = m_milliseconds * 10 + (c - '0');
}
else if(!isdigit(c))
{
m_error = true;
Expand Down Expand Up @@ -1049,6 +1055,10 @@ class ISO_8601BasicDateParser : public DateParser
m_state = 7;
stateStartIndex = index + 1;
}
else if (isdigit(c) && (index - stateStartIndex) < 3)
{
m_milliseconds = m_milliseconds * 10 + (c - '0');
}
else if (!isdigit(c) || index - stateStartIndex > 3)
{
m_error = true;
Expand Down Expand Up @@ -1447,6 +1457,7 @@ void DateTime::ConvertTimestampStringToTimePoint(const char* timestamp, DateForm
{
std::tm timeStruct;
bool isUtc = true;
int milliseconds = 0;

switch (format)
{
Expand All @@ -1466,6 +1477,7 @@ void DateTime::ConvertTimestampStringToTimePoint(const char* timestamp, DateForm
m_valid = parser.WasParseSuccessful();
isUtc = parser.ShouldIAssumeThisIsUTC();
timeStruct = parser.GetParsedTimestamp();
milliseconds = parser.GetParsedMilliseconds();
break;
}
case DateFormat::ISO_8601_BASIC:
Expand All @@ -1475,6 +1487,7 @@ void DateTime::ConvertTimestampStringToTimePoint(const char* timestamp, DateForm
m_valid = parser.WasParseSuccessful();
isUtc = parser.ShouldIAssumeThisIsUTC();
timeStruct = parser.GetParsedTimestamp();
milliseconds = parser.GetParsedMilliseconds();
break;
}
case DateFormat::AutoDetect:
Expand All @@ -1495,6 +1508,7 @@ void DateTime::ConvertTimestampStringToTimePoint(const char* timestamp, DateForm
m_valid = true;
isUtc = isoParser.ShouldIAssumeThisIsUTC();
timeStruct = isoParser.GetParsedTimestamp();
milliseconds = isoParser.GetParsedMilliseconds();
break;
}
ISO_8601BasicDateParser isoBasicParser(timestamp);
Expand All @@ -1504,6 +1518,7 @@ void DateTime::ConvertTimestampStringToTimePoint(const char* timestamp, DateForm
m_valid = true;
isUtc = isoBasicParser.ShouldIAssumeThisIsUTC();
timeStruct = isoBasicParser.GetParsedTimestamp();
milliseconds = isoBasicParser.GetParsedMilliseconds();
break;
}
m_valid = false;
Expand All @@ -1529,6 +1544,7 @@ void DateTime::ConvertTimestampStringToTimePoint(const char* timestamp, DateForm
if (IsSecondsSinceEpochRepresentable(tt))
{
m_time = std::chrono::system_clock::from_time_t(tt);
m_time += std::chrono::milliseconds(milliseconds);

@kai-ion kai-ion Sep 22, 2026

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Nit/minor: Time is assigned by comparing to how long it has been since 1970, an edgecase of date before 1970 would render a negative number. if m_time is negative, % 1000 on a negative number stays negative.

Fix: floor when splitting into seconds + ms so the fraction is always 0–999

@sbiscigl sbiscigl Sep 22, 2026

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not sure if this is a valid case, can you create a string such that a negative value for a millisecond exists? the state machine should take care of that.

}
else
{
Expand Down
46 changes: 46 additions & 0 deletions tests/aws-cpp-sdk-core-tests/utils/DateTimeTest.cpp
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Expand Up @@ -264,6 +264,20 @@ TEST_F(DateTimeTest, TestISO_8601ParsingNSPrecision)
ASSERT_EQ(gmtDate, DateTime(utcOffsetVersion, DateFormat::ISO_8601));
}

TEST_F(DateTimeTest, TestISO_8601ParsingPreservesMilliseconds)
{
DateTime gmtDate("2026-09-15T18:00:00.016Z", DateFormat::ISO_8601);
ASSERT_TRUE(gmtDate.WasParseSuccessful());
ASSERT_EQ(16, gmtDate.Millis() % 1000);

@sbiscigl sbiscigl Sep 22, 2026 •
edited
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shouldn't gmtDate.Millis() be 16? and not gmtDate.Millis() % 1000? 2026-09-15T18:00:00.016Z means that theres .016 seconds on the time meaning 16 miliseconds. shouldnt we not need the modulo operation on it? let me know if im mistaken Millis in this context is epoch millis not millis of datetime

}

TEST_F(DateTimeTest, TestISO_8601MillisecondRoundTrip)
{
DateTime gmtDate("2026-09-15T18:00:00.016Z", DateFormat::ISO_8601);
ASSERT_TRUE(gmtDate.WasParseSuccessful());
ASSERT_EQ("2026-09-15T18:00:00.016", gmtDate.ToGmtStringWithMs());
}

TEST_F(DateTimeTest, TestISO_8601ParsingTooLowPrecision)
{
const char* gmtDateStr = "2002-10-02T08:05:09.12Z";
Expand Down Expand Up @@ -296,6 +310,20 @@ TEST_F(DateTimeTest, TestISO_8601BasicParsingMSPrecision)
ASSERT_EQ(gmtDate, DateTime(utcOffsetVersion, DateFormat::ISO_8601_BASIC));
}

TEST_F(DateTimeTest, TestISO_8601BasicParsingPreservesMilliseconds)
{
DateTime gmtDate("20021002T080509016Z", DateFormat::ISO_8601_BASIC);
ASSERT_TRUE(gmtDate.WasParseSuccessful());
ASSERT_EQ(16, gmtDate.Millis() % 1000);
}

TEST_F(DateTimeTest, TestISO_8601BasicMillisecondRoundTrip)
{
DateTime gmtDate("20021002T080509016Z", DateFormat::ISO_8601_BASIC);
ASSERT_TRUE(gmtDate.WasParseSuccessful());
ASSERT_EQ("2002-10-02T08:05:09.016", gmtDate.ToGmtStringWithMs());
}

TEST_F(DateTimeTest, TestISO_8601Parsing_DOS_Stopped)
{
const char* gmtDateStr = "Weddkasdiweijbnawei8eriojngsdgasdgsdf1gasd8asdgfasdfgsdikweisdfksdnsdksdklasdfsdklasdfdfsdfsdfsdfsadfasdafsdfgjjfgghdfgsdfsfsdfsdfasdfsdfasdfsdfasdfsdf";
Expand Down Expand Up @@ -380,3 +408,21 @@ TEST_F(DateTimeTest, TestFormatAutoDetect)
DateTime parsedBadDate(badDate, DateFormat::AutoDetect);
ASSERT_FALSE(parsedBadDate.WasParseSuccessful());
}

TEST_F(DateTimeTest, TestAutoDetectPreservesMilliseconds)
{
const char* isoMsDate = "2026-09-15T18:00:00.016Z";
DateTime parsed(isoMsDate, DateFormat::AutoDetect);
ASSERT_TRUE(parsed.WasParseSuccessful());
ASSERT_EQ(16, parsed.Millis() % 1000);
ASSERT_EQ(DateTime(isoMsDate, DateFormat::ISO_8601), parsed);
}

TEST_F(DateTimeTest, TestAutoDetectBasicPreservesMilliseconds)
{
const char* basicMsDate = "20021002T080509016Z";
DateTime parsed(basicMsDate, DateFormat::AutoDetect);
ASSERT_TRUE(parsed.WasParseSuccessful());
ASSERT_EQ(16, parsed.Millis() % 1000);
ASSERT_EQ(DateTime(basicMsDate, DateFormat::ISO_8601_BASIC), parsed);
}
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