I would have thought the answer was obvious just from looking at the two dates. Doesn’t it seem apparent to you that startDate of 2018-04-27 is the date value stored internally as 0 UTC, but userStartDate of 26/Apr/18 (2018-04-26) is that same date, but represented in the user’s local format AND time zone, which includes any offset to UTC configured for Jira.
Maybe try some experiments yourself to try and isolate the cause of the problem, such as:
- Look at the dates via the GUI to see if they also exhibit the same ‘apr’ / ‘abr’ behaviour
- Find if the problem happens for every single version in every single project
- Find if the problem changes when you change Jira’s timezone or date settings
- Find if the problem changes when you make test versions and start / release them at different times
- Find if the problem only affect dates in April, or every other month too
- Ask one of the other devs on your team to give you their interpretation of the problem