Prior to version 1.3.0, activity times were interpreted in the local time zone. If your software recorded in UTC, it was assumed that this was your local time zone. This assumption was often wrong. With version 1.3.0, this got fixed. Activities will be imported using UTC or specified time zone. They will then be localized into the time zone of the activity position.
Use this tool to fix activities which were imported with an older version.
Your activities are shown in the local time zone of the activity but no time zone is shown.
Interpret times as local time zone
Your activities seem to be UTC and don't conform to the local time zone. You have the impression that all the times are off.
Everything seems broken. You want to reload all your activities. Be aware that with Strava API that can take ages due to the rate limiting. Also be aware that this will delete all changes that you did after importing the activity (changing the name, equipment, adding tags, trimming the activity).
Throw all activities away and rescan