The basic SI unit for time is the second. Its duration is defined as a 9,192,631,770 periods of the radiation of the transition between the two hyperfine levels of the ground state of the cesium133 atom. It was originally defined based on a day.
Other units of time include the minute which is 60 seconds, the hour which is 60 minutes or 3600 seconds, the day which is the time needed for the sun to return to its highest in the sky or about 24 hours, the week which is a period of 7 days, the month which is based originally on orbital period of the moon and varies from 28-31 day and the year which is based on the orbital period of the Earth as it moves around the Sun and is a period of 365.2596 days. The week is the only time unit not based on an astronomical period being based on the Jewish Sabbath cycle which is based on the creation week of Genesis 1. The calendar year is rounded to 365 days with a leap day every 4 years.
According to Special and General Relativity time is a 4th dimension. It is unique in that we only travel through time one way. (forward).
Time is one of the three main parts of the Universe: space, time and mass. Relativity shows them to be inter-related with space and time are so interrelated that they form a single entity. Mass warps space-time forming gravity and motion of mass contracts both space and time.
The perception of the rate of time is variable but time is always experienced in the same way. We remember the past, we exist in the present and we look forward to the future. In our common experience time is the sequence of events we experience. Physics has shown that it is to be a lot more. It is a fundamental part of the universe.