The following is a list of date stamps by year for your reference. *Taken from TPF*

years 1997-present are encased in a Square; after 2014 may or may not be in a square
years 1971-1996 are encased in a Circle
years 1945-1970 have no geometric shape

2015 T no square or circle surrounding it
2014 R
2013 Q
2012 P
2011 O
2010 N
2009 M
2008 L
2007 K
2006 J
2005 I
2004 H
2003 G
2002 F
2001 E
2000 D
1999 C
1998 B
1997 A* letters began being encased in a Square
1996 Z
1995 Y
1994 X
1993 W
1992 V
1991 U
1990 T
1989 S
1988 R
1987 Q
1986 P
1985 O
1984 N
1983 M
1982 L
1981 K
1980 J
1979 I
1978 H
1977 G
1976 F
1975 E
1974 D
1973 C
1972 B

1971 A* letters began being encased in a Circle