Remittance deadline lookup
Pick the month participant contributions were withheld and your plan’s size, and read the DOL’s own timing rule for when that money has to reach the plan: the same standard and outer limit /check and /method already cite.
The general standard, every plan size
“the assets of the plan include amounts... that a participant has withheld from his wages by an employer, for contribution... to the plan, as of the earliest date on which such contributions... can reasonably be segregated from the employer's general assets.”
29 CFR 2510.3-102(a)(1)
Full mechanics, including why the 15th business day is a ceiling and not a grace period, and the regulation’s own definition of a business day, are in When is a 401(k) deposit legally late? The rule sponsors misread. This tool is independent research, not legal or tax advice.