网上写As long as your Solo 401(k) was setup before December 31s
As long as your Solo 401(k) was setup before December 31st, you may make contributions as an “employer” for the previous year until the IRS tax deadline. 不知道我的情况是否适用,我没有注册公司,是用SCHedule C报税
https://www.sharebuilder401k.com/blog/solo-401k-tax-deadlines-and-how-you-can-minimize-taxes/