Are "y", "m", and "d" the singular and plural abbreviations for "year(s)", "month(s)", and "day(s)"?
First, is it the right abbreviation? No. The common abbreviation is to use "yr" for year. As for the plural, we commonly use "yrs".
Now to your examples. They're both different.
In the first sentence, you'd write the full words like this.
The project took exactly 5 years, 2 months and 18 days to complete.
As for the second sentence, it would be written as:
It was a 5-year, 2-month and 18-day project.
You've said It was 'a', so you must use the singular form.
Now to the big question... Is your usage acceptable? Well, there two things I'd see here:
- Context
- Information format
If for example, you want to write this format of y, m and d in a computer-generated report, there's no problem. It saves on the ink and is understandable.
However, if you're writing this method in an English assignment, it wouldn't be a good idea to do so.
With any abbreviation, you're trading compactness and speed for aural equivalence.
Audience and context should be considered to avoid confusion. This means in general, one should use the most "generally accepted" abbreviations.
For years and months that is yr. and mo. You could be more compact with 2 yr. or less (but more equivalent) with 2 yrs./mos.
In context, nobody is going to think 5y 2m 18d means 5 years, 2 meters, and 18 days.
You would, however, get into trouble if you were to write "We don't have enough wood and will need 2m before we can complete the project."
Most important of all:
If your boss likes a certain format, USE THAT FORMAT.
These abbreviations are frequently used as placeholders for the individual digits of the date.
For example: an online form asking for your birthdate. Might have MM/DD/YY (or DD/MM/YY outside of the US) with a pull down menu or type in field to add those numbers.
I wouldn't call them the proper abbreviations, but I don't think anyone would have trouble understanding their use.