Any dates are better than none. Unfortunately I wasn't able to find Harold or Hazel under any spelling coming close to to Grellman in the 1920 or 1930 census. Without more information it will be hard to make any headway. And surprisingly Nels Nelsen is more common than you would think.
I did find a Harold E. Grellman who was born Oct 16, 1911 and died June 22, 1992 in Ganges, Allegon Co., Michigan. I believe he married L. Marie Binkley on Oct 13, 1931 in Cook Co., IL. If this looks like the right Harold I suggest trying to get his obituary.
Do you have info on who Hazel married and when and where she died?