Is that a bug? I interpret it as from midnight 2016-04-04 to the first time which has the date 2019-01-31. That includes the full day of 2016-04-04 but nothing from 2019-01-31. This is contrasted with the opposite direction which is midnight 2019-01-31 going backwards, thus not including 2019-01-31 and all the way up till the first time which is 2016-04-04 non-inclusive.
Both show 1032 days. Do space missions depend on such fuzzy defined time stamps as (calendar?) months? I think time dilation would be a much more serious factor.
https://www.wolframalpha.com/input?i=2016-04-04+to+2019-01-3... = 2 years 9 months 27 days
https://www.wolframalpha.com/input?i=2019-01-31+to+2016-04-0... = 2 years 9 months 26 days
The duration should be identical but is off by one day.
One day a space mission will fail due to this bug.
Reported several times, they never cared.