ews: calculate offset from timezone for local time only

It is only necessary to calculate the offset if the client sends
datetime values using local time and provides timezone information in
separate tags. For UTC times, it is not necesary to calculate the
offset.

Author: gromandreas

exch/ews/context.cpp

@@ -1813,13 +1813,16 @@ void EWSContext::toContent(const std::string& dir, tCalendarItem& item, sShape&
 		shape.write(TAGGED_PROPVAL{PR_MESSAGE_CLASS, deconst("IPM.Appointment")});
 	int64_t startOffset = 0, endOffset = 0;
 	time_t startTime = 0, endTime = 0;
+	bool calcStartOffset, calcEndOffset = false;
 	if(item.Start) {
 		startTime = clock::to_time_t(item.Start.value().time);
 		startOffset = std::chrono::duration_cast<std::chrono::minutes>(item.Start.value().offset).count();
+		calcStartOffset = item.Start.value().needCalcOffset();
 	}
 	if(item.End) {
 		endTime = clock::to_time_t(item.End.value().time);
 		endOffset = std::chrono::duration_cast<std::chrono::minutes>(item.Start.value().offset).count();
+		calcEndOffset = item.Start.value().needCalcOffset();
 	}
 	// TODO handle no start and/or end times
 
@@ -2035,17 +2038,20 @@ void EWSContext::toContent(const std::string& dir, tCalendarItem& item, sShape&
 				shape.write(NtAppointmentTimeZoneDefinitionEndDisplay,
 					TAGGED_PROPVAL{PT_BINARY, temp_bin});
 
-				// If the offsets of start or end times are not set, probably
-				// the client didn't send the offset information in date tags.
-				// Try to get the offset from the timezone definition.
-				if(startOffset == 0 || endOffset == 0)
+				// If the offsets of start or end times are 0 and
+				// the client didn't send the offset information in date tags,
+				// try to get the offset from the timezone definition.
+				if((startOffset == 0 && calcStartOffset) || (endOffset == 0 && calcEndOffset))
 				{
 					EXT_PULL ext_pull;
 					TIMEZONEDEFINITION tzdef;
 					ext_pull.init(buf->data(), buf->size(), alloc, EXT_FLAG_UTF16);
 					if(ext_pull.g_tzdef(&tzdef) != EXT_ERR_SUCCESS)
 						throw EWS::DispatchError(E3294);
-					startOffset = endOffset = offset_from_tz(&tzdef, startTime);
+					if(calcStartOffset)
+						startOffset = offset_from_tz(&tzdef, startTime);
+					if(calcEndOffset)
+						endOffset = offset_from_tz(&tzdef, endTime);
 				}
 				item.Start.value().offset = std::chrono::minutes(startOffset);
 				item.End.value().offset = std::chrono::minutes(endOffset);

exch/ews/structures.cpp

@@ -1194,6 +1194,24 @@ sTimePoint::sTimePoint(time_point tp, const tSerializableTimeZone& tz) :
 /**
  * @brief     Create time point from date-time string
  *
+ * As of 2025-01-27 we have identified 3 different datetime formats sent by
+ * the clients.
+ * Zulu time (UTC), no DST, no offset:
+ * `<t:Start>2024-04-29T19:30:00Z</t:Start>`
+ * With the offset information in datetime, additionally a timezone tag:
+ * `<t:Start>2024-11-27T13:00:00+01:00</t:Start>`
+ * `<t:StartTimeZone Id="W. Europe Standard Time"/>`
+ * `<t:EndTimeZone Id="W. Europe Standard Time"/>`
+ * Local time with a timezone tag, it's necessary to calculate the offset:
+ * `<t:Start>2024-09-25T09:00:00</t:Start>`
+ * ```
+ * <t:ExtendedProperty>
+ *   <t:ExtendedFieldURI PropertyName="CalendarTimeZone" PropertySetId="A7B529B5-4B75-47A7-A24F-20743D6C55CD" PropertyType="String"/>
+ *   <t:Value>Europe/Vienna</t:Value>
+ * </t:ExtendedProperty>
+ * ```
+ * `<t:MeetingTimeZone TimeZoneName="W. Europe Standard Time"/>`
+ *
  * @throw     DeserializationError   Conversion failed
  *
  * @param     Date-time string
@@ -1218,6 +1236,8 @@ sTimePoint::sTimePoint(const char* dtstr)
 	time = clock::from_time_t(timestamp);
 	time += std::chrono::duration_cast<time_point::duration>(std::chrono::duration<double>(seconds));
 	offset = std::chrono::minutes(60*tz_hour+(tz_hour < 0? -tz_min : tz_min));
+	if(strlen(dtstr) == 19)
+		calcOffset = true;
 }
 
 /**
@@ -1232,6 +1252,12 @@ sTimePoint sTimePoint::fromNT(uint64_t timestamp)
 uint64_t sTimePoint::toNT() const
 {return rop_util_unix_to_nttime(time-offset);}
 
+/**
+ * @brief     Whether it's necessary to calculate the offset from timezone
+ */
+bool sTimePoint::needCalcOffset() const
+{return calcOffset;}
+
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 // Types implementation
 

exch/ews/structures.hpp

@@ -452,9 +452,11 @@ struct sTimePoint
 
 	static sTimePoint fromNT(uint64_t);
 	uint64_t toNT() const;
+	bool needCalcOffset() const;
 
 	time_point time{};
 	std::chrono::minutes offset = std::chrono::minutes(0);
+	bool calcOffset = false;
 };
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////