Datetime Time-saving Facts

Quick pointers on the formats and standards to be aware when working with date and time data.

Epoch Time (Unix Time/Posix Time)

The number of seconds elapsed since 01 Jan 1970 midnight
Integer value without timezone. i.e., it represents UTC timestamp.
Always increasing with every passing second.
Typically, number of seconds. Also used are number of milliseconds and nanoseconds for greater precision.
Most scalable/robust way of representing datetime in DB.

Current global standard time. Historically, known as GMT (Greenwich Mean Time).
De facto for International communication, navigation, scientific research, and commerce.
Clock time at reference time line (longitude passing Greewich city, UK).
Has no timezone (since it's 0th timezone meridian).
Every other world clock is derived from UTC time, by adding/subtracting timezone offsets (+/- hh:mm).
UTC offset for Indian Standard Time (IST) is +05:30.
Z character in ISO datetime format indicates UTC, i.e., without timezone.
DB stores datetime in UTC.
Always, choose UTC in business logic/service layers. Timezone conversion should be handled at presentation layer.

International standard for communicating/representing Date and Time related data.
Greatest temporal term (typically a year) is placed at the left, followed by each successively lesser term.
YYYY-MM-DDThh:mm:ss.sssZ, where optional timezone offset Z for UTC, or +/-hh:mm for others.
String format of datetime, if required (in DB or JSON), should ideally be in ISO.

Mean Solar Time - Time calculation based on the position of Sun in the sky, where "day" is the fundamental unit of solar time.
Leap Second - "one second" adjustment that's occasionally added to UTC time to accommodate for difference between precise time and observed Solar Time.
Atomic Clock - Most accurate clock, where time is measured by monitoring resonant frequency of atoms.
Daylight Saving Time (DST) - The practice of advancing clocks to make better use of the longer daylight available during summer so that darkness falls at a later clock time.