Rudolf Carnap, in an appendix to his Introduction to Symbolic Logic, sets out various different models of time. He describes them in terms of different languages for describing and reasoning about time. I clearly remember my delight when I first came across his account of the subject, as I had never really thought much about different ways of describing time.

Different models of time:

• Continuous flow models
• Discrete event models
• Intermittent events - time is when something happens
• Regular events - time is when the clock (atom, quark whatever) ticks
• Cyclical models (the Buddhist approach), bounded models (the Judeo-Christian approach), open ended models (big bang followed by a descent into entropy) 

I often look at an image and wonder, what model of time does it represent? 

You may (or may not) be interested to know that there have been approximately 12,918,272,375,533,807,200 nanoseconds since 1st Jan 1601. I think I prefer a continuous model.