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:
1900 1906 1917 1923 1934 1945 1951 1962 1973 1979 1990 2001 2007 2018 2029 2035 2046 2057 2063 2074 2085 2091
1901 1907 1918 1929 1935 1946 1957 1963 1974 1985 1991 2002 2013 2019 2030 2041 2047 2058 2069 2075 2086 2097
1902 1913 1919 1930 1941 1947 1958 1969 1975 1986 1997 2003 2014 2025 2031 2042 2053 2059 2070 2081 2087 2098
1903 1914 1925 1931 1942 1953 1959 1970 1981 1987 1998 2009 2015 2026 2037 2043 2054 2065 2071 2082 2093 2099
1904 1932 1960 1988 2016 2044 2072
1905 1911 1922 1933 1939 1950 1961 1967 1978 1989 1995 2006 2017 2023 2034 2045 2051 2062 2073 2079 2090
1908 1936 1964 1992 2020 2048 2076
1909 1915 1926 1937 1943 1954 1965 1971 1982 1993 1999 2010 2021 2027 2038 2049 2055 2066 2077 2083 2094 2100
1910 1921 1927 1938 1949 1955 1966 1977 1983 1994 2005 2011 2022 2033 2039 2050 2061 2067 2078 2089 2095
1912 1940 1968 1996 2024 2052 2080
1916 1944 1972 2000 2028 2056 2084
1920 1948 1976 2004 2032 2060 2088
1924 1952 1980 2008 2036 2064 2092
1928 1956 1984 2012 2040 2068 2096


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.


I got the idea for this while watching the Simpsons.
When Marge wanted Homer to throw out some old calendars he replied,

Sure, it's not 1985 now, but who knows what tomorrow will bring?




And then I thought, "well wait a minute, those calendars might actually work again some day."