The international style for dates is day of the week (optional), day, month and year without any punctuation or ordinals (3rd, 1st, 7th).

Tuesday 23 June 2015
23 June 2015

In US English and the media, the month comes before the day and you need a comma to separate the numbers.

Tuesday June 23, 2015

Dates are often abbreviated in informal text (e.g. emails) and in tables were space is limited. Most abbreviations of the first few letters take full stops (e.g. Inc.), but the full stops have dropped off many common abbreviations. Whether you use full stops for some common abbreviations is now a style choice.

Mon 8 Aug
or
Mon. 8 Aug.

Dates in digits
When writing dates as digits, the conventional sequence is day, month and year separated by forward slashes or full stops. Most style guides recommend two digits.

20/01/05
20.01.05

Some organisations give the century as well as the year.

20.01.1950
20/01/1950

Decades and centuries
When you are referring to a year span within a decade or the same century, you don’t need to repeat the century figures. Use an en dash ( – ) or hyphen ( – ) to separate the numbers.

1993–98 (en dash)
2001-25 (hyphen)

You don’t need an apostrophe when referring to decades.

Fashions were amazing in the 1970s.
The 1970s fashion was amazing.

Financial years
Different styles for writing financial years are:

financial year 2015
financial year 2014/15
financial year 2014–15 (en dash)
financial year 2014-15 (hyphen)

Different styles for abbreviated financial years are:

FY2014/15
FY2014–15 (en dash)
FY2014-15 (hyphen)
FY2015
FY15