Federal servants who want to sue their employer for discrimination under Title VII must contact the EEOC "“within 45 days of the date of the matter alleged to be discriminatory.” 29 CFR §1614.105(a)(1)." A constructive discharge claim is one in which the employee technically quits - but only because (s)he faced intolerable discrimination in the workplace. Courts generally treat a constructive discharge like a firing.
When does a claim like that accrue (triggering the 45-day period)? You have a few choices... the date of the last discriminatory act; the effective date of the resignation; or the date of notice of resignation. We have an answer (bragging time: I totally called it!):
[W]e hold that a constructive-discharge claim accrues—and the limitations period begins to run— when the employee gives notice of his resignation.Off the top of my head, I can't think of any reason why this analysis would not hold for analyzing the running of the statute of limitations in similar discrimination claims against private employers (but candidly admit that I have not had occasion to fully research that issue).