Dates and times vary over different regions of the world and in response to adjustments to match solar time. The primary time standard is called Coordinated Universal Time (also known as UTC). However, local time standards can be affected by:
The unix time stamp is a way to track time as a running total of seconds. This count starts at the Unix Epoch on January 1st, 1970 at UTC.
DST is Daylight Saving Time, an adjustment of the timezone by (usually) one hour during part of the year. DST rules are determined by local law and can change from year to year.
The world is divided up into about 24 time zones. A time zone represents a region where the local time is some fixed offset from a global reference (usually UTC), or a time zone can represent a region throughout which the local time is always consistent even though the offset may fluctuate seasonally.
When converting zone time to or from UTC, dates must be properly taken into account.
For example, 10 September at 02 UTC is the same as 9 September at 21 EST (U.S.).
Similarly, Japan - which uses Japanese Standard Time (JST) - is nine hours ahead of UTC, which is represented as "+09:00". So at noon UTC it is "210000+09:00" in Japan.