Styles exist within the publishing world. You don’t have to adopt these conventions but, if you do, it may help you to be more consistent in your writing.
If it’s necessary to specify an exact time, you have two options:
Note that a colon is used in 10:30 but a full point in 10.30am.
Note also that the second option begs the question whether to write a.m. or am! With full points to indicate an abbreviation (ante meridian = a.m.) or without? Which looks cleaner on the page? Your choice!
Times that are not so precise are usually spelled out:
Dates can be expressed in a number of ways:
Notice the use of a comma in the second option.
It’s not good practice to abbreviate the names of months; write them in full.
Ordinals (including the ‘st’, ‘nd’ or ‘th’) should be used if the month is implied rather than stated.
Notice that ‘th’ is not presented as a superscript.
The year number can be given as a number except when it appears at the start of a sentence. (You should never start a sentence with a number.)
Year numbers can be abbreviated, provided the century is unambiguous.
However, unless the abbreviated form is a recognised title, ask yourself : Why abbreviate?
When referring to a single decade, there are two options: as a number or in words.
For the first two decades of an entire century, I prefer to use numbers; ‘aughts’ and ‘teens’ seem too clumsy for me.
Notice there is no apostrophe in 1970s; it’s a plural, not a possessive.
You could abbreviate:
For ten-year ranges, again, there are options: 2001-2009 or 2001-09. Never 2001-9.
Whatever you decide, be consistent!
For names of centuries, there are two options:
Notice that the ‘c’ is lower case.
With AD and BC (or BCE and CE), AD precedes the year while BC follows it, and you might present these in small caps.