No, you cannot rely on the 15th day of the month following for a timely deposit, no matter what size you are.
The deposit of participant contributions must be made on the earliest date on which such contributions or repayments can reasonably be segregated from the employer's general assets. (§2510.3-102 (a)(1))
This means, plain and simple, that you have to make the deposit as soon as you are able. In most cases this will mean the same day it is withheld from the employee’s paycheck. There can sometimes be legitimate reasons why the deposit cannot take place on the same day or maybe the day after. In those cases, a facts and circumstances test will determine whether the delay was reasonable and the deposit couldn’t have been made earlier than it was.
There is an “outside window” or maximum time period for making deposits. This maximum time period is the 15th day of the month following separation from the employee’s paycheck. This does not mean that as long as you make it by the 15th of the month following you are ok. It means that even if a tornado swept up your HR person and dropped them on the yellow brick road, the deposit will never be timely if made after the 15th of the month following. The exact language is
You are stuck with late deposits that you now need to correct. Don’t panic though, the correction is a fairly straight forward process. You calculate the lost earnings owed to the participants, pay a small excise tax, and possibly file with the DOL. A good practitioner will be able to do this for you for a reasonable fee.
Does your plan attach an audit with it’s Form 5500? If so, I can guarantee you that the independent auditor will not sign off on the audit without correction.