Ignoring Time Zone Issues Can Have Serious Consequences
According to the bug report in DateTime Difference Bug Makes PayPal_Framework Impractical for Real-Time Transactions, the fact that different components of a particular software system reside in different time zones--and use their own local time for timestamps--resulted in a critical error.
Timestamps should be recorded in UT (Universal Time, which most people incorrectly refer to as either GMT or UTC,) especially in situations where system components may reside in different time zones.
Even when all server and client machines will be in the same time zone, recording timestamps in local time can cause bugs.
For example, in most of the United States, whether 2005-10-30T01:50:42 is greater than or less than 2005-10-30T01:10:17 (both local time) is not well defined, because both timestamps occur during the ambigous hour T01:00:00/T02:00:00 (a left closed, right open interval) which occurs twice in local time on days where a transition from DST to standard time occurs. Most of the United States transitioned from DST to standard time at T02:00:00 local time on 30 October 2005--which resulted in local time in San Franciso (for example) jumping from 2005-10-30T01:59:59-07:00 to 2005-10-30T01:00:00-08:00.
Such discontinuities and ambiguities do not occur when timestamps are recorded in UT. For example, the UT timestamp that corresponds to 2005-10-30T01:59:59-07:00 is 2005-10-30T08:59:59Z, and the UT timestamp that corresponds to 2005-10-30T01:00:00-08:00 is 2005-10-30T09:00:00Z.
Don't add to the bug list.
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