A number which often appears on the panel or in the margin of a map. Take the last two digits of the last four numbers and invert them, e.g., 3680 1209R = 1990. Do not confuse with the stock number, which is also in the same vicinity. (Source: David Cobb, Harvard Map Collection)

Examples:
1744 = 1947 ["74" reversed is "1947"]
218412 = 1948
20353 = 1953
1354 = 1953
C-11558 = 1955
15658 = 1956
C-15857 = 1958
C-1859 = 1958
C-5361 = 1963

After this date they seem to use more standard dating, e.g., 9-67 = 1967.

To obtain the publication date on maps using the Buddhist calendar (used, for example, in Thailand), subtract 542 from the date shown on the map.

Examples:
2516 = 1974 (2516 - 542 = 1974)
2519 = 1977

Dates Based on Addresses
? - 1910 Pacific Bldg., San Francisco
1911-1914 681 Market (Monandock Bldg.), San Francisco
1915-1925 1628 Van Ness, San Francisco
1926- 150 Van Ness, San Francisco

Date Codes
3-28 = 1928
526 = 1925 (Reverse first 2 digits for year)
728 = 1927
053 = 1950
154 = 1951
3601 = 1963
After 1965 dates expressed as 4-66 = 1966, 6-68 = 1968.

Clason Map Company was a map publisher in Denver, Colorado that published the first road atlas of the U.S. and Canada. It failed during the Great Depression. Clason maps are notorious for having no date information. Instead, various elements of the maps changed over time. Use the link above to connect to a site at Michigan State University to learn more about dating Clason maps.

Diversified Maps

The following information was provided by Jim Wakefield:

Some, but not all, Diversified maps contianed a three or four digit date code.  It was often, but not always in the bottom left or bottom right corner of the body of the map.  If the code contains three digits, the first digit appears to reference the base map.  If the code contains four digits, the first two digits reference the base map.  The second to last digit (which, in my experience, is always a 1 or a 2) appears to stand for a first or a second edition of the year of issue.  Finally, the last digit of the code is the last digit of the year of issue.

Diversified appeared to have started producing maps in 1962 and stopped production in the early 70's.  However, it did not start using this code until 1964.  (I have not been able to find date codes on maps issued by Diversified in 1962 or 63).  Therefore, it is not too hard to figure out the year of a map containing the single digit date code.. Here's an example:  I have a 1964 Standard Oil map of Minnesota produced by Diversified.  It contains the code 1424.  The 14 references Diversified's base map of Minnesota; the 2 suggests that the map is a second edition for the year of issue; and finally the 4 indicates that the year of issue is 1964.

One final note: Diversified's use of its code was spotty, at best. Sometimes it was used and sometimes it wasn't.  Therefore, the presence of this code on a Diversified map will allow you to date the map. However, the absense of the code does not mean the map was necessarily issued in one of the years before Diversified began using its code. Hope this info, confusing as it is, is helpful to you.

Founded December 1, 1909 as: Engineering and General Draughting Company. In 1933, General Drafting Company was the first company to produce four-color road maps. In May, 1952, the company moved from New York City to Convent Station, N.J.

Examples:
850 - August, 1950
52-5-52 - [1952?]
47-1-53N - [1953?]
657W - [1957?]
W158 - 1958
W460-A8 - 1960
W863-A8 - 1963

Source: New York Public Library Map Division, March 1989. Revised May 23, 1992.

Dates on George F. Cram maps are preceded by the letters CR. These letters are then followed by a number that denotes the month and year that the map was published. Thus a map of Baltimore which has the following numbers on it: N132 CR654 was published in June of 1954.

Source: Phil McDaniel at George F. Cram Company

In or near the border at one corner of each map [atlases are coded differently] is a coded reference to the date of revision. Substitute numbers for the following letters to get the month and year:

A - 0
B - 9
C - 8
D - 7
E - 6
F - 5
G - 4
H - 3
I - 2
J - 1

Example:
S78 = May 1978

Source: Biblio (University of Illinois Map and Geography Library), 1987-1.

At the bottom left corner of each map (usually) are some letters. If you substitute numbers for letters in the world CUMBERLAND to get month and year the map was published.

C - 1 U - 2
M - 3
B - 4
E - 5
R - 6
L - 7
A - 8
N - 9
D - 0

Example:
L.BR = 7th month of [19]46

Source: ACMLA Bulletin 40, p. 57.

Road Map Collectors Association date code list (1927-1978).

Date Codes for Later H.M. Gousha Maps, 1979-1996, by Ken Rockwell

The website for the Road Map Collectors Association (linked above), contains a list of Date Codes used by Rand McNally and H.M. Gousha Maps. Sometimes this code is the only indication of the date. This list ends with the year 1978. Below is a table containing the two-letter date codes for maps published by H.M. Gousha during the last three decades of that company's existence. During much of this period, an explicit statement of date appears somewhere in the map, such as in the legend. This allows for construction of a list of proper dates. 

For earlier decades, Gousha indicated the year by a single letter (A-Z), then by double-letters (AA-ZZ, the last being from 1978). After that, the two-letter code became more complex. It started of logically enough, with AB and AC, then became quite irregular in pattern, for reasons known only to whoever assigned these codes. The form for the entire code varies as well: sometimes the two-letter code is the first element, sometimes a numeral precedes it. For example, on a sheet with separate maps of Arizona and New Mexico, the codes read "11-AN-588-J" and "AN-2-598-J," respectively.

The following list gives the two-letter date code, the year it indicates, and an OCLC number for a record input by the Library of Congress. Occasionally the map code on a given map doesn't agree with its publication date, but this is usually a case of a reprint. For example, a street map of Denver (OCLC #27850962) bears the statement "1992 edition," but also has a 1991 copyright date, so the two-letter date code for 1991 is used.

The codes for the last three years are "reversals" of the codes for 1981-1983. I assume that if there had been any maps published by Gousha in 1997, the code would read "LY." Unfortunately, by then the esteemed old breakaway from Rand McNally had been re-assimilated into the "mother company."

Year	Date Code	OCLC #
1979	AB	7691663
1980	AC	7009136
1981	AN	8728484
1982	MZ	19100488
1983	OB	9969262
1984	YL	11532703
1985	CP	12945104
1986	KX	14718116
1987	QD	16980033
1988	WJ	33442193
1989	ER	21668644
1990	IV	21668685
1991	SF	24545649
1992	UH	35887082
1993	GT	31412545
1994	NA	32092773
1995	ZM	35556250
1996	BO	34823801

See Gregory F. Christiano's web site: My Recollection: The Hagstrom Map Company Publication Code.

Contributed by: Nancy Kandoian, New York Public Library.

The following information was suppied by Angie Cope, Cartographic Materials Catalog Librarian, American Geographical Society Library, University of Wisconsin-Milwaukee Libraries

1926-34: Date may be determined from the first digit following the first letter of the code.  There are usually 2 codes:  one on the cover, and one on the map inside.  Use the later date if they don't agree.

Examples:
R0529 (cover), Q08CE (inside) = 1930
2P8818 (cover), O716C (inside) = 1928

Also:
Maps with text only covers are from 1926-29; illus. covers 1930-34.
Maps measuring 5" x 7" usually 1926-28; 5-1/4" x 8-1/2", 1927-34.
Maps titled "Highway Map and Guide of [state] are from 1926-33.
Maps titled "Highway Map [state] are from 1934.

Examples:
6-37 = 1937
12442 = 1942
40557 = 1957
24160 = 1960
50761 = 1961
508384 = 1983/84 [i.e., published in 1983]

Small code along the bottom of the map on the inside of the neat line.

Examples:
310-17 = 2003 January - map 17  (1st and 3rd digits reversed = year, i.e., 2003; 2nd digit = month, i.e., January)
3110-17 = 2003 November - map 17  (1st and 4th digits reversed = year, i.e., 2003; 2nd and 3rd digits = month, i.e., November)

Source: Angie Cope "NovoPrint USA, Inc. Maps and their Codes," WAML Information Bulletin v. 34 no. 3 (July 2003), p. 118.

The Road Map Collectors Association has a web page on Dating Rand McNally and H.M Gousha Maps.

The Map List Help Page from USGS has information on dates used on USGS topographic maps.