There are many different ways to measure the passing of time, whether by watching the shadow cast by a stick in the mud, or building complex machines made of thousands of different parts that can measure the change of one moment to the next by the smallest imaginable degree. Time can be measured using the cyclic change of any natural, biological or artificial process. A time standard is any officially recognised method of doing so.
Time standards can be local, regional, national or global in their reach. Most modern standards measure time in terms of seconds, minutes, hours, days, months and years. While even the most primitive animals can differentiate between day and night, the first time standards were based on the Earth's rotation. Astronomers would gaze up at the sun and the stars and use their changing positions and the recurring seasons to plot the rotation of the earth.
History of standardised time
For a long time throughout history it was considered that the Earth rotated at a constant speed, but in the 19th century scientists learned, from studying records of eclipses, that the rotation of the Earth is actually slowing down.