That information will not be anywhere on a tax return. Your W-2 and/or tax documents only show how much you earned during a calendar year--not the specific dates. You might be able to check with Social Security for a work history--not sure if that will show specific start and end dates for employment.
WORK HISTORY https://www.ssa.gov/forms/ssa-7050.pdf
Or---contact that old employer and just ask them.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
