Is there one word that has the meaning of "at that time and prior to that"?
For example, "2018 and prior to 2018". The purpose of asking this question originally was that I needed to name 3 categories or folders - "pre-1999 (inclusive)", "2000 to 2010", "post 2011 (inclusive)" - for a project where different materials would be placed in them accordingly. I still would like to simplify the first and last ones.
Unfortunately, English is notoriously ambiguous when it comes to terms around time. There is no one word that encompasses both of these definitions, and the only short phrase I can think of is "up to and including". Example:
The country was stable up to and including most of 2018, but a series of public scandals in the summer of that year rocked faith in the government, plunging the entire country into chaos.
Almost every other term, such as until or up to is still ambiguous, as it does not specify how much of the event took place within the given year. You'd have to add more information to be specific. For example:
Until 2018 the singer was relatively unknown, but a hit Christmas single in 2017 pushed her into the spotlight, and soon it seemed everyone knew her name.
In this example the exact timing of the singer's fame is ambiguous, and indeed, unimportant. The point is that, more or less, she was not famous in 2017, and was famous in 2018.
That being said, you can add the word "inclusive" to a statement to mean "including the start and end points of the set". Example:
The test gave us one minute to add up the numbers between 1 and 100, inclusive.
However, this term is mostly used in things like science and mathematics, and can sound a little strange in casual conversation, or with dates. Unless of course, the conversation or the dates is about some kind of mathematics.
The revenue for the company from 2010 to 2015, inclusive, was only a fraction of what it made in 2016.
I would rephrase it as follows:
2018 and prior to 2018.
→ 2018 and before.
From Merriam-Webster:
2 : at an earlier time
// the night before
// knew her before
In a comment under your question, you added:
Group A has the employees who joined the company prior to 2000 (up to December 31, 2000). Employees in Group B joined the company from 2001 or after (starting January 1, 2001)
Note that December 31, 2000 is in 2000, so you can't say prior to 2000 because that would exclude the 2000 dates they were there. (You could say prior to 2001, however.)
This could be paraphrased as:
Group A has employees who joined the the company in 2000 and before, and Group B has employees who joined the company in 2001 and after.
In addition to the and before / and after pairing of words, another possibility is and earlier / and later. They carry mostly the same meaning, and which is used is generally a matter of personal opinion.
Use "until" if something happened in 2018 that changed things, but it was true before 2018 and in part of 2018.
https://www.myenglishpages.com/site_php_files/grammar-lesson-prepositions-time.php
