How does the brain keep track of time?

A: The brain keeps track of time in different ways at different time scales.

Fast time interval estimation (sub-second) is important for timing complex behavior and playing musical instruments. Slow time interval estimation may be important for planning the day, although daylight and hunger can provide cues. Intervals greater than a day may rely on observing the repetition of day-night cycles and seasonal changes.

The following diagram shows the accuracy of time interval estimation for humans and other animals, along with proposed neural mechanisms for each time scale. Slow time scales (hours) are at the top and fast scales (milliseconds) are at the bottom. Each diamond represents the result of one scientific study. [1]

There are multiple competing models of time perception with no final consensus yet on which mechanisms the brain uses in which situations. [2]

Related:
How does the human brain distinguish similar events that occur at different times?

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[1] Buhusi CV, Meck WH (2005). What makes us tick? Functional and neural mechanisms of interval timing. Nature Reviews Neuroscience. (http://scholar.google.com/schola…)

[2] Ivry RB, Schlerf JE (2008). Dedicated and intrinsic models of time perception. Trends in Cognitive Sciences. (http://scholar.google.com/schola…).