It's neither people use centuries and decades every where
It is customary.
A "gran" is a German measurement, it is neither customary or metric. A "gram" is a metric unit.
metric and customary
metric
They are both systems of measurement except that the metric system is simpler and used by the majority of the world.
No a quart is a customary measurement for the capacity of liquid as where a Kilometer is a metric measurement for length
using classical rather than metric units of measurement
There are thousands of measurement units in both systems and it is not possible to list them all.
No. Century is a measurement of time. A century is 100 years
It is a customary unit.
Myanmar formerly known as Burma uses the imperial system of measurement. It used to be a British colony and has not yet adopted the metric system.
The opposing systems are imperial and metric, not customary. But, meters are metric.