If you have some other dates close to the death, you could make an estimated death date or range.
For example, if you knew the person wrote their will on Jan. 1, 1900 and the will was proven on Aug. 15, 1902, you could enter the death as "1 Jan 1900 - 15 Aug 1902".
Similarly, if you find them on the 1930 Census, and find their widow on the 1940 Census, you could enter "1930-1940".
If you know that this person was buried on May 5, 1950, you could enter "Bef. 5 May 1950".
If you have absolutely no idea what to enter, you can simply enter "Deceased" or "Dead" into the date field.
For example, if you knew the person wrote their will on Jan. 1, 1900 and the will was proven on Aug. 15, 1902, you could enter the death as "1 Jan 1900 - 15 Aug 1902".
Similarly, if you find them on the 1930 Census, and find their widow on the 1940 Census, you could enter "1930-1940".
If you know that this person was buried on May 5, 1950, you could enter "Bef. 5 May 1950".
If you have absolutely no idea what to enter, you can simply enter "Deceased" or "Dead" into the date field.