Reminders take into account the time zone of a current user profile:
When a user creates a reminder for 7 PM the app assumes that it is 7 PM in the current user time zone.
When a user browses the list of reminders they see reminder dispatch time according to their current time zone.
For the application to work correctly, it is essential that you make sure that your time zone in Jira settings matches the actual time zone of your location.
Your current time zone is displayed to the right above the "My reminders" table.
Case 1. Anna lives in New York (GMT -5 in her profile) and creates a reminder for Peter to 9 AM. Peter lives in London (GMT is in his profile) so Anna's 9 AM is his 2 PM. They will see the same reminder dispatch time differently.
Case 2. Anna creates a reminder to herself to 9 AM. She goes on a business trip and her new time zone is -1h to the NY time. She changes the time zone in her profile – then she will see her reminder planned for 8 AM.
Note that changing your timezone will not affect other users or the existing reminders dispatch time, it will only affect how the date-time information is being displayed for you in the Jira interface.