Here's a breakdown:
Restrictive Time Expressions:
* These are essential to understanding the meaning of the sentence. They provide specific information about when something happened.
* No comma needed.
Examples:
* "The meeting will start at 10:00 AM."
* "I finished the project by Friday afternoon."
* "We arrived on Tuesday."
Non-Restrictive Time Expressions:
* These provide additional, non-essential information. The sentence would still make sense without them.
* Comma needed before the time expression.
Examples:
* "The meeting, which was scheduled for 10:00 AM, was delayed."
* "We finished the project on time, by Friday afternoon."
* "The team arrived, on Tuesday, to begin work."
Here are some additional points:
* If a time expression is part of a longer phrase, the comma may go before the entire phrase.
* In some cases, you might use a comma before a time expression for stylistic reasons or to improve clarity, even if it's not strictly necessary.
If you're unsure, it's always best to err on the side of caution and use a comma if it's optional.