19) Write out the month, preceded by a figure for the day. Use four digits when referring to specific years.
15 March 1928
Never write a date according to either of the following structures:
- ..the 15th day of March 1928 (written out)
- the fifteenth day of March 1928 (even worse)
- 15th March 1928 (ordinal day indication)
- March 15 1928 (without comma)
- March 15th, 1928 (combination ordinal indication and reverse order)
- 09 March 1928 (zero for days below 10)
Only exceptionally, in footnotes, table headers or small columns where space is precious, the month can be written as a number. Therefore, avoid an all-figure style of writing dates. Example:
- not 15-3-1928
- 1966 not ’66
Other than in the EU and most other countries, in the United States it is more common to write March 15, 1928 and have the year also followed by a comma. It would even be appropriate to write 3.15.28. In the (ISO) standardised international dating system it is 1928-03-15 (note that days and months below 10 are preceded by a zero). Obviously, whichever dating system you use, do it consistently.
If only the month and year are indicated (or a particular day with a year, as in New Year’s Day 2009), neither the European nor the American system inserts a comma.
In letter agreements, if the relevant year is absolutely clear from the context, the year number might be left out:
We refer to the Master Collaboration Agreement dated 18 May 2009 between our companies (the MCA), and the first meeting of the steering group on 31 October. This letter is to confirm that the feasibility study referred to in Article 3.3(a) of the MCA…
20) When referring to decades write the 1980s (without apostrophe).
However, a reference to the first two decades of a century should be rephrased (a reference to the 2000s is ambiguous since it might also be interpreted as a reference to the entire 21st century and a reference to the 2010s just does not work – compare its expressed equivalent the teens).
21) When referring to a period of time use an en-dash closed-up with the years.
For the second figure, do not repeat the century if it is the same, but always include the decade:
1928-35, 1990-96, 1996-2006, 2010-12
However, note the following deviations in running texts:
- from 1990 to 1995 not from 1990-95
- between 1990 and 1995 not between 1990-95
- 1990 to 1995 inclusive not 1990-95 inclusive
- since 2003 not 2003-present
The same principle applies to references to the first decade of a new century: the century should be repeated for the first decade: 2003-2009.
It is important to note that 1993-94 is two years (and 1993-98 six). Single years (e.g., financial or academic years) that do not coincide with the calendar year should be denoted by a slash: 1993/94, which is twelve months (or less)!
 This best practice rule is derived from EC DG Translation, English Style Guide – A handbook for authors and translators in the European Commission, revised edition 2009.
 § 6.46 The Chicago Manual of Style, The essential guide for writers, editors, and publishers, The University of Chicago Press, 15th edition 2003.