There are 43,200 seconds in 12 hours
At a 20 second difference per hour over 24 hours, the difference will be 480 hours per day.
If the clock gets off by 480 seconds in a day it will take 90 days to get off by 12 hours, and thus back in sync.
If they set the clocks on Jan 31st, then during a non-leap year, the time until May 1st would be 90 days (28 in Feb + 31 Mar + 30 Apr + 1 May).
Leap year happens every 4 years, and it adds an extra day to that total (91 days). Therefore, it is impossible for the clocks to be set in January, and realign in May, during a leap year.
Additionally, 1932 was a leap year. This means that whomever was born in 1933 would turn 47 in 1980 (another leap year, as the difference between 1980 and 1932 is 48, which is a multiple of 4).
IF Robert were born in 1933, he would be turning 27 on a leap year. As previously established, we cannot make it from Janurary to May within 90 days on a leap year. Therefore, the current year must be 1979, and Robert must be born in 1932.
Thus, Robert is older than Arthur, who was born in 1933.