Learn English – How to write “all-in-one” correctly

There are only two kinds of documents in current usage that spell out the year -- legal documents, and wedding invitations.

Legal documents normally spell the year in lowercase, such as in this numbing bit of prose from West Virginia:

For any tax imposed under the provisions of this article with respect to any taxable year prior to the first day of January, one thousand nine hundred eighty-three, a resident individual shall be allowed a West Virginia exemption of six hundred dollars for each exemption for which he is entitled to a deduction for the taxable year for federal income tax purposes.

Some legal documents capitalize everything for extra pomp, but it's uncommon:

In testimony whereof, I hereunto subscribe my hand and affix the seal of said Court, at Office, in Nashville, the 6th day of December in the Year One Thousand Nine Hundred Eighty-Four and in the 209th Year of American Independence.

Wedding invitations, as noted, tend to capitalize the first letter only; however, this seems to be the only justification I can find:

The “T” in Two thousand doesn’t have to be capitalized but everyone does it so it would probably look incorrect if it wasn’t and it will look more polished if it is capitalized.

So, in modern usage, it appears that the rules for capitalization are:

• Spell the year out in lowercase.
• Except in wedding invitations, where the first letter is capitalized because everyone does it.

You would not capitalize "standard" because it is not a proper noun.

Think about it this way, if you were to be taking about a city named "San Franciso" rather than a standard named ISO 1923801: The Proper Use of Capital Letters you would not capitalize later instances of the word "city."

I visited the city of San Franciso. I found the city to be delightful.

If you wanted to be very clear in your text, you can give the work an alias, using parentheses.

For example:

I spent a lot of time on the popular standard on capitalization ISO 1923801: The Proper Use of Capital Letters ("the Standard"). Further perusals of the Standard continued to be useful.

You will find this type of device more often in legal documents.