“By what means the concept of Calendars and Time started?” .A Calendar is a time-keeping device in which the year is divided into months, weeks, and days. Calendar days are units of time based on the earth’s motion relative to the sun. A Tropical year contains 365.2422 days and in order to make it more usable, civil year of 365 days is considered. What about the remaining 0.2422 days? Julius Caesar introduced Leap year to avoid this discrepancy by adding extra one day to February whenever the year is divisible by 4. This forms the Julian calendar, in which the civil year was 365.25 days on average, approximately equaling tropical year. Universal time coordinated (UTC) is the time used for all time keeping purposes, and it is the time reference which is broadcast by the National Bureau of Standards as a standard for setting clocks. It is based on the Atomic time-frequency standard. The fundamental unit of UTC is the mean solar day. Mean solar day is divided into 24h, an hour into 60 min, and a minute into 60s. There are 86,400 clock seconds in a mean solar day. UTC is similar to Greenwich Mean Time (GMT) where fraction of a second is not taken into consideration.