3. Stored date/time values

Issue

Many computer systems need to represent future dates and times. A calendar system where you can schedule meetings and other events is a good example. Depending on how the system stores the date/time values, there may be a need to make corrections to accommodate the new DST rules.

In particular, if the system stores the date and time as a combined value that is relative to Universal Time (UTC), then the data in the system may be off by an hour for the parts of the year that used to be in standard time but are now in daylight saving time. An example may help explain why changes are needed:

Let’s say you’re in the Eastern time zone. If you entered a repeating meeting for 9:00 am every Monday, the system would store it as 14:00 UTC for weeks outside of DST and 13:00 UTC for weeks inside DST. The system takes care of the translation to UTC for you, so you might not even be aware that it’s happening. If the old rules were in effect when you entered the weekly meeting, it would use the 14:00 UTC values for late March dates. Under the 2007 DST rules, those need to be stored as 13:00 UTC in order to show up at 9:00 am Eastern. The recorded events in your system need to be adjusted back one hour.

Recommendation

For third-party software, contact the vendor to find out if the product is affected by the DST change. Systems that store date/times relative to the local time zone are not likely to be impacted but products that support multiple time zones will often require remediation. For solutions developed in-house, consider the places where date and time values are stored as one field in a format relative to UTC.

Impact

Systems that store date/time values relative to UTC will display incorrect times within the three weeks in spring and one week in fall that are now part of DST. In particular, the time shown will be one hour later than intended. This will affect events not only in 2007, but in any future year as well.

Some calendar systems store day-long events internally as midnight to midnight records. These may get shifted by one hour during the new DST weeks, causing the events to extend into the day after they were intended to finish.

Other unexpected behavior may occur with calendar systems that synchronize events between two or more types of devices, such as a computer and a smart phone. The exact symptoms will depend on how each devices stores date/time values and how those values are represented by the synchronization software.