The 3921 Form deadline for issuing forms to participants is January 31st. The deadline to file 3921 Forms with the IRS is February 28th if filing on paper, or March 31st if filing electronically. If the deadline date falls on a Saturday, Sunday or legal holiday, the deadline would be the next business day.
For more information, Form 3921 instructions can be found at https://www.irs.gov/pub/irs-pdf/i3921.pdf