Why does my activity show the wrong date on Strava?
In most cases, an incorrect date is due to a recording error on your device or two different activities joined together in one file. GPS devices are known to occasionally malfunction and record data either in the past or in the future. Also, two different activities can be uploaded as one if the device was not reset between recordings.
How to fix your activity's start date:
First, go to the Strava website to view the activity where you can check the total elapsed time. If the total elapsed time is a very large number, then the GPS device has recorded data on different days. If you believe this is a simple case of two activities recorded together as one, use the split activity tool. Or, if only a few points at the beginning or end of your file have the wrong date, consider trimming this data with the activity crop tool. Garmin devices are known to malfunction by recording some part of the activity in the future. There is a useful website called fitfiletools.com that can repair Garmin files with date errors. Lastly, you can export the GPX file from Strava, open in any text editor, and find and replace all the incorrect dates with the correct dates. If you're recording with the Strava mobile app, check that you have the phone's clock set to update automatically.
If an activity has the wrong start time on Strava:
Check your profile settings and make sure your location is set properly. This can affect what timezone is registered on your Strava account. Activities recorded indoors with no GPS data will default to the timezone of the location listed on your profile.