Geography is not only concerned with space and neither is GML. The temporal.xsd schema (see http://schemas.opengis.net/gml/3.1.1/base/temporal.xsd  ) contains a set of useful definitions for time related objects and properties. The simplest of these is TimeInstant.
A TimeInstant is what it says – an instant in time. It has a single child (property) called timePosition. A TimeInstant is thus written:
Of course not all times are instants – some are TimePeriods. For these we can write:
Of course GML can provide a number of ways of expressing time durations and time lengths as well and can deal with non-ISO times and dates (need a Geological Calendar?) but that is beyond our scope for now.