Retirement tax questions

Thank you for your reply, @JulieS .  The plan has been in place for many years. 

 

My understanding is that I must formally elect to make an employee deferral contribution by Dec. 31., but the the actual contribution can be made up until the tax-filing deadline.  My question is: what does it mean to "formally elect"?  How do I substantiate that I've done so?

 

Ryan