Is 0% considered a deferral election?

Can someone please tell me if my interpretation is correct?  A SH 401(k) has dual entry, January 1/July 1, with deferral modifications allowed any payroll period.  If an employee becomes eligible to participate on July 1 and chooses not to contribute at that time then changes their mind in September, can they start contributing immediately or do they have to wait until the next entry date of January 1?  I consider the election not to contribute as a deferral of 0% which is then subject to the deferral modification dates and they can contribute the next payroll period.

They entered the plan on July and became a participant.

You state that participants can modify election "on any payroll period" so they can start on the next payroll after they turn in their election subject to admin procedures on when changes need to be made to be implemented. That is payroll might need some time to enter it into the system.

Perfect, thank you very much!