This question was discussed in a Communities with one of the Portfolio UX designers in a thread titled how to edit start end date epic jira portfolio:
Portfolio for JIRA helps you do agile planning. Hence it's not really meant for planning with start end end dates as such, as it bases it's decisions off of other things.
You can assign issues to a sprint or release to place them roughly in a given time frame and then the scheduling algorithm will calculate their actual placement and duration based on ranking, estimation, child issues, capacity of the team, and many other signals.
Take a look at how to edit start end date epic jira portfolio for a bit more information.
I was also facing the same issue where everything was looking fine and still the scheduled end date of a particular issue was showing wrong(There was no reason of showing it a date which was far ahead).
I did some rnd and did something which solved my issue. I opened the ticket and found that in the "Fix Version/s:" field there was a value. This value was the name of 'release' I created on JIRA. As per my understanding this field should be empty for this particular issue as I had not released any thing on this. So, I simply removed the entry and worked for me.
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