The following is an excerpt in English translation from the beginning of a traditional Orthodox and Conservative ketubah taken from this site:
On the ______day of the week, the _________day of the month ______ in the year five thousand seven hundred and ______ since the creation of the world
The bolded phrase is what is called Anno Mundi which assigns the number of years from the creation of the world. This phrasing seems to be the most common usage in ketubot.
However, would it be legal (a valid ketubah) if someone decided to use a different system of counting? For example, instead of listing the Anno Mundi year, which is currently 5776, someone used the Machzor kattan counting. For this year it would be the 19th year of the 304th cycle (based on the 19 year Metonic cycle that is used for positioning Judaic leap years). Or what about if they used the secular date (2016)?
Note: If using the Gregorian date (or Julian, as well), the entire date would need to be changed to a transliterated form that would correspond to say, "June 7, 2016". (Transliteration is already commonly used in the ketubah when mentioning the town / city name.) You would also eliminate the "creation of the world" phrase and substitute, perhaps, (AD or CE)