User Guide
7.5. Time Zones
If you travel across the planet, time zones can be confusing. If you get a capture file from some-
where around the world time zones can even be a lot more confusing ;-)
First of all, there are two reasons why you may not need to think about time zones at all:
• You are only interested in the time differences between the packet time stamps and don't need to
know the exact date and time of the captured packets (which is often the case).
• You don't get capture files from different time zones than your own, so there are simply no time
zone problems. For example: everyone in your team is working in the same time zone as your-
self.
What are time zones?
People expect that the time reflects the sunset. Dawn should be in the morning maybe around
06:00 and dusk in the evening maybe at 20:00. These times will obviously vary depending on
the season. It would be very confusing if everyone on earth would use the same global time as
this would correspond to the sunset only at a small part of the world.
For that reason, the earth is split into several different time zones, each zone with a local time
that corresponds to the local sunset.
The time zone's base time is UTC (Coordinated Universal Time) or Zulu Time (military and
aviation). The older term GMT (Greenwich Mean Time) shouldn't be used as it is slightly in-
correct (up to 0.9 seconds difference to UTC). The UTC base time equals to 0 (based at
Greenwich, England) and all time zones have an offset to UTC between -12 to +14 hours!
For example: If you live in Berlin you are in a time zone one hour earlier than UTC, so you
are in time zone "+1" (time difference in hours compared to UTC). If it's 3 o'clock in Berlin
it's 2 o'clock in UTC "at the same moment".
Be aware that at a few places on earth don't use time zones with even hour offsets (e.g. New
Delhi uses UTC+05:30)!
Further information can be found at: http://en.wikipedia.org/wiki/Time_zone and ht-
tp://en.wikipedia.org/wiki/Coordinated_Universal_Time.
What is daylight saving time (DST)?
Daylight Saving Time (DST), also known as Summer Time, is intended to "save" some day-
light during the summer months. To do this, a lot of countries (but not all!) add a DST hour to
the already existing UTC offset. So you may need to take another hour (or in very rare cases
even two hours!) difference into your "time zone calculations".
Unfortunately, the date at which DST actually takes effect is different throughout the world.
You may also note, that the northern and southern hemispheres have opposite DST's (e.g.
while it's summer in Europe it's winter in Australia).
Keep in mind: UTC remains the same all year around, regardless of DST!
Further information can be found at: http://en.wikipedia.org/wiki/Daylight_saving.
Further time zone and DST information can be found at: http://wwp.greenwichmeantime.com/ and
http://www.timeanddate.com/worldclock/.
Advanced Topics
138