W3C Date/Time profile of ISO8601
The Digital Commonwealth repository system requires that dates be formatted using w3cdtf encoding. Standardization of date entry will allow the system to parse the information into a human-readable display date.

w3cdtf encoding requires that dates be recorded as follows:

Year: YYYY (e.g., 1872)
Year and month: YYYY-MM (e.g., 1872-11)
Complete date: YYYY-MM-DD (e.g., 1872-11-09)

To express a date range, record the start and end dates separately. For example, an item created on November 9 and 10, 1872 would be recorded as:

Date start = 1872-11-09
Date end = 1872-11-10

If a date or date range requires a qualifier (e.g., “circa 1872”) or can only be inferred, the numeric dates would be recorded and the appropriate qualifier from the list below would be used.

Qualifiers:

approximate: Used to identify dates that have been approximated and may not be exact, such as circa dates (e.g., “ca. 1872”).

questionable: Used to identify questionable dates (e.g., “1872?”).

inferred: Used to identify dates that have not been transcribed directly from the resource, but have been inferred from another source (e.g., “[1872]”).

Note that the date used to describe the digital surrogate should be the date on which the item was originally created or issued, not the date on which it was digitally recreated.

Non-numeric dates formats: Sometimes physical items will be notated with dates such as “Early 1850,” “19th century,” Summer 1907” (or similar). This style of dating will not fit into the w3cdtf encoding model. In such instances, the textual dates should be replaced by numeric date ranges (using “start” and “end” dates as described above).

Though by no means exhaustive, the following list provides examples of how one might approach date reconfiguration in certain instances.

Centuries:
For centuries, use xx00 – xx99.  For example, 19th century = 1800 – 1899 (approximate), 20th century = 1900 – 1999 (approximate)

For early centuries, use xx00 – xx39. For example, early 19th century = 1800 – 1839 (approximate), early 20th century = 1900 – 1939 (approximate)

For mid centuries, use xx30 – xx69. For example, mid 19th century = 1830 – 1869 (approximate), mid 20th century = 1930 – 1969 (approximate)

For late centuries, use xx60 – xx99. For example, late 19th century = 1860 – 1899 (approximate), late 20th century = 1960 – 1999 (approximate)

Decades:
For decades, use xxx0 – xxx9. For example, 1970s = 1970 – 1979 (approximate), 1850s = 1850 – 1859 (approximate)

For early decades, use xxx0 – xxx3. For example, early 1970s = 1970 – 1973 (approximate), early 1850s = 1850 – 1853 (approximate)

For mid decades, use xxx4 – xxx6. For example, mid 1970s = 1974 – 1976 (approximate), mid 1850s = 1854 – 1856 (approximate)

For late decades, use xxx7 – xxx9. For example, late 1970s = 1977 – 1979 (approximate), late 1850s = 1857 – 1859 (approximate)

Parts of years (early, mid, late):

Early part of year = Jan, Feb, Mar, Apr. For example, early 1970 = 1970-01 to 1970-04 (approximate), early 1850 = 1850-01 to 1850-04 (approximate)

Mid part of year = May, June, July, Aug. For example, mid 1970 = 1970-05 to 1970-08 (approximate), mid 1850 = 1850-05 to 1850-08 (approximate)

Late part of year = Sep, Oct, Nov, Dec. For example, late 1970 = 1970-09 to 1970-12 (approximate), late 1850 = 1850-09 to 1850-12 (approximate)

Seasons:
Winter = Dec, Jan, Feb. For example, winter 1970 = 1969-12 to 1970-02 (approximate), winter 1855 = 1854-12 to 1855-02 (approximate)

Spring = Mar, Apr, May. For example, Spring 1970 = 1970-03 to 1970-05 (approximate), spring 1855 = 1855-03 to 1855-05 (approximate)

Summer = Jun, Jul, Aug. For example, Summer 1970 = 1970-06 to 1970-08 (approximate), summer 1855 = 1855-06 to 1855-08 (approximate)

Fall = Sep, Oct, Nov. For example, Fall 1970 = 1970-09 to 1970-11 (approximate), fall 1855 = 1855-09 to 1855-11 (approximate)