"In each of the examples above, a different epoch is implied. Sometimes an epoch has meaning for several millennia. Other times the meaning of an epoch is lost after a while (such as the start of a timer, or when the computer booted). However, if two time_points are known to share the same epoch, they can be subtracted, yielding a valid duration, even if the definition of the epoch no longer has meaning.

In Boost.Chrono, an epoch is a purely abstract and unspecified concept. There is no type representing an epoch. It is simply an idea that relates (or doesn't) time_points to a clock, and in the case that they share a clock, time_points to one another. time_points associated with different clocks are generally not interoperable unless the relationship between the epochs associated with each clock is known."

From http://www.boost.org/doc/libs/1_48_0/doc/html/chrono/users_guide.html

My understanding is that chrono isn't meant to provide absolute time "position", but time measurement and manipulations.