In most cases, this parameter is not necessary. Some regions of the world do not recognize the beginning of daylight savings time on the first Sunday in April. If your server is in a region where this is true, use this parameter to specify the exact date when DST begins. Use this setting along with DST_End_Date, which specifies when daylight savings time ends.
See the settings DST_End_Date and DSTlaw for additional ways to adjust the time stamp for daylight savings.