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# 2.3.2 Calculating Dates and Times

The ActiveSync protocols use the UTC time and the originator's time zone for all meetings. For single occurrences, the device converts the time to the local time zone. The originator's time zone is not important because the original conversion to UTC accounts for time zone and DST. However, for recurring meetings, there is the possibility of a transition into or out of DST during the series. The stored UTC corresponds to the first occurrence of the series, but later meetings can have different corresponding UTC times. Therefore, to display the correct time, the device performs one calculation that accounts for the originator's time zone, in addition to the device's local time zone.

The following table shows the time zone information for the earlier examples.

 Time zone information Pacific Time Mountain Time (Arizona) Eastern Time Time zone offset UTC-8 UTC-7 UTC-5 Daylight start 4/6/03 02:00 None 4/6/03 02:00 Daylight end 10/26/03 02:00 None 10/26/03 02:00 Daylight bias +1 0 +1

The calculation to display the local time of a meeting instance is as follows:

(Meeting time in UTC) + (local time zone offset) + (local daylight bias) – (original daylight bias)

Note: Daylight bias is a time zone's offset during DST. The local daylight bias comes from the local time zone information, and the original daylight bias comes from the originator's time zone information.

The weekly conference call repeats every Friday beginning 4/4/03. The start time of the first instance is 10:00 A.M. PT, or 18:00 UTC. Therefore, the stored time is 18:00 and the time zone is Pacific Time.

 Date Seattle Arizona New York 4/4/03 1800+(-8)+(0)-(0) = 1000 1800+(-7)+(0)-(0) = 1100 1800+(-5)+(0)-(0) = 1300 4/11/03 1800+(-8)+(+1)-(+1) = 1000 1800+(-7)+(0)-(+1) = 1000 1800+(-5)+(+1)-(+1) = 1300

Notice that both the local and original DST biases are the ones in effect on the date/time of the meeting instance.

The weekly conference call repeats every Friday beginning on 4/4/03. The originator was in Arizona, so the start time of the first instance is 11:00 MST (Arizona), or 18:00 UTC. The stored time is 18:00 and the time zone is MST (Arizona).

 Date Seattle Arizona New York 4/4/03 1800+(-8)+(0)-(0) = 1000 1800+(-7)+(0)-(0) = 1100 1800+(-5)+(0)-(0) = 1300 4/11/03 1800+(-8)+(+1)-(0) = 1100 1800+(-7)+(0)-(0) = 1100 1800+(-5)+(+1)-(0) = 1400
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