You can recycle your old calendars by using them again!

For the year 2012,

the following old calendars will work:

1984

1956

1928

Here are the next 10 years, and equivalent prior-year calendars going back to 1900:

2011: 2005 1994 1983 1977 1966 1955 1949 1938 1927 1921 1910 2012: 1984 1956 1928 2013: 2002 1991 1985 1974 1963 1957 1946 1935 1929 1918 1907 1901 2014: 2003 1997 1986 1975 1969 1958 1947 1941 1930 1919 1913 1902 2015: 2009 1998 1987 1981 1970 1959 1953 1942 1931 1925 1914 1903 2016: 1988 1960 1932 1904 2017: 2006 1995 1989 1978 1967 1961 1950 1939 1933 1922 1911 1905 2018: 2007 2001 1990 1979 1973 1962 1951 1945 1934 1923 1917 1906 1900 2019: 2013 2002 1991 1985 1974 1963 1957 1946 1935 1929 1918 1907 1901 2020: 1992 1964 1936 1908 2021: 2010 1999 1993 1982 1971 1965 1954 1943 1937 1926 1915 1909

Here are all years 1900 through 2100. Every row contains years which share a calendar:

Here is the Java source that generated these values.

Basically it uses java.util.Calendar to determine if two years are equivalent by looking at the first and last days of the year.

If two years begin with the same day of the week, and end with the same day of the week, I assume they are equivalent.