Ever wonder we have Eastern (or Mountain, or Pacific) Standard Time? You can thank the railroads. On November 18, 1889 railroads in the United States began using the set of "Standard" timezones that we more or less use today. The color blobs in this figure show the timezones used today around the world. Before standard time each community kept track of time. Some important times (such as noon) where announced by ringing bells or another signal. Imagine a train arriving in one town before it left the last one! Not everyone was happy and some towns continued to use local solar time until 1918.
Today we release SDO marked in Coordinated Universal Time (diplomatically called UTC) and International Atomic Time (similarly, TAI). TAI is the number of seconds since midnite January 1, 1958. A series of laboratories keep track of the march of time. UTC maps TAI to almost local solar time at the Greenwich Meridian in England (the line where longitude is 0). This means that UTC has leap seconds to keep up with the slowing down of the Earth's rotation. Right now we have added 34 leap seconds to UTC. We like TAI because it is easy to do differences in time by subtracting the TAI times. This is not true for UTC.
When you look an SDO timestamp it will say Z or UTC if the time is UTC; T or TAI when it is TAI.