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\label{sec:TimeInterval}
A TimeInterval represents a period between time instants. 
It can be either positive or negative.

There are TimeInterval methods defined for setting and getting 
a TimeInterval, for printing the contents of a TimeInterval.

The class used to represent time instants in ESMF is Time, 
and this class is frequently used in operations along with 
TimeIntervals.  The Clock class, for example, advances model time by 
incrementing a Time with a TimeInterval. 

TimeIntervals are used by other parts of the ESMF timekeeping 
system, such as Clocks; see Section~\ref{sec:Clock}.
