It'd be kinda annoying if they could have it one way for the WARN act and it was another way for the H1B.
EDIT: This apparently is wrong, see downthread; the original text below is preserved for context
When they are no longer legally employed, with legal employment including any terminal leave. (But not extended by severance, even if that is calculated based on pay for a particular time period.)
But also it's not clear, and it might be the legal employment end date in some cases.
1. http://blog.cyrusmehta.com/2022/11/guide-to-terminated-nonci...
If USCIS denies your application, then the 60 day clock starts the moment you stopped working (for Twitter laid off folks, this has already started). If USCIS approves your application, then the 60 day clock begins the moment you stopped being an employee of the company (for Twitter laid off folks, this is January 4).