The field of clinical natural language processing (NLP) has been built on the analysis of clinical sublanguage characteristics. It is well recognized that not only does clinical sublanguage differ from general English (or other languages) but also clinical sublanguage differs among clinical subspecialties and among corpora originating from different healthcare systems. A less recognized aspect is that clinical sublanguage, like all languages, evolves over time. This paper analyses the evolution of clinical sublanguage using a large, national clinical text corpus spanning 15 years. Through the analyses of document types, length, ngrams, and concepts, we found strong evidence that clinical sublanguage does evolve and such changes have implications for NLP development and maintenance. Although the analysis is performed on one corpus, our observations of sublanguage changes are generalizable.