With many countries that are not close to the equator, a time adjustment is made to the Local Time each spring and autumn, to adjust the time forward or backward, typically by one hour. This time adjustment is used to make the most of the smaller number of daylight hours over the winter period and to save artificial energy. This time adjustment is known as Daylight Saving Time (DST).
In the UK, for example, Greenwich Mean Time (GMT) is the Local Time between 01:00 on the last Sunday in October and 01:00 on the last Sunday in March. For the rest of the year, the Local Time is UTC+1 (known in the UK as ‘British Summer Time’ (BST)).
On the eastern coast of America, many of the US states observe Eastern Daylight Time (EDT) during the summer months and Eastern Standard Time (EST) during the winter months. With EDT, the Local Time is 4 hours behind UTC; with EST, the Local Time is 5 hours behind UTC (or UTC-4 and UTC-5 respectively). So, for example, an event that occurred at 10:07:49.000 EST would be logged internally in ClearSCADA as having occurred at 15:07:49.000 UTC.
For devices that run in Local Time and apply Daylight Saving Time adjustments, the most noticeable affect of the twice yearly time change is in the amount of data stored around the time that the clocks change (see The Effects of Daylight Saving Time Adjustments).
Be aware that not all devices that run in Local Time support time adjustments in relation to Daylight Saving Time (DST). The relevant configuration section mentions if any such limitations exist.