You are correct, this is likely due to timezone differences. The Iteration dates follow the Workspace timezone. If individual users have a timezone set in their user profiles that differs from the Workspace timezone, the data will be displayed based on the user's profile settings and the data is offset to correlate with the timezone for the Workspace.
If users would like to avoid this in the future, they may edit the timezone in their user profile settings to either match the Workspace timezone or they can select "Use Default" in the timezone field which will match up to the Workspace timezone.