Complex sentence: For example, one may write to mean "1981 April" or to mean "April 5", and one may simply write "1981" to refer to that year or "19" to refer to the century from 1900 to 1999 inclusive.
Split into simpler sentences: For example, one may write to mean "1981 April". One may simply write "1981" to refer to that year or "19" to refer to the century from 1900 to 1999 inclusive.