There are many examples of coins being made years, even decades, after their stated date. In 19th and 20th century Spain, one date was adopted for each coin, that date was used year after year. The actual year of minting was hidden inside tiny stars on the coin. In a number of Muslim countries, the year in which a ruler came to the throne was used on coins year after year. The actual year the coin was made was indicated by another number placed somewhere on the coin. The Ottoman sultan Abdul Hamid II came to the throne in 1293 AH (1876) and ruled till 1909 (1327 AH). In Egypt, his coins all carry the date 1293, with an additional number (1 through 33) indicating the actual year of his reign in which the coin was made.
In 1949 all three U.S. mints struck dollar coins under contract with China. The coins were not US dollars (not being made at that time), but Chinese dollars dated 1934 and Mexican dollars dated 1898. Mexico also struck 1898 dated pesos that year for China. Some Chinese silver coins dated 1909-1911 were actually struck or restruck for circulation in later years, into the 1920's. Some undated Chinese dragon dollars, which were originally made during 1890-1911, were restruck for circulation in 1949. All of the Szechuan 5 Cash coins dated 1912 were actually made in 1918 and 1919.
During the Southern Sung Dynasty in China (1100's -1200's) coins were dated with the year of the emperor's reign -- 1, 2, 3, etc. One emperor died in his 6th year and a new emperor took the throne (that year being year 1 of the new emperor). However some mints which were far away on the other side of the country, didn't receive the news in time, and actually made and circulated coins dated 7th year --- a year which did not exist!