There are several reasons your activity appears to start at the wrong time or on the wrong date. If an activity doesn't contain GPS data, check that your time zone is set correctly in your profile settings. Some devices include time zone information for non-GPS activities, but if yours doesn't, Strava defaults to the time zone listed on your profile. Occasionally, Strava can get stuck and fail to update this automatically.
You can adjust your timezone using these instructions, which should resolve the issue for future uploads.
On the Strava website, open the dropdown menu under your profile picture, then select Settings.
Replace your current location with Los Angeles > Save > Refresh Page.
Enter your correct location > Save > Refresh Page. Confirm that your time zone is now set properly.
Unfortunately, the only way to fix the timestamps on your existing activities is to re-upload them. This will require deleting any existing versions of that activity so the new version(s) can be uploaded. This will remove all kudos, comments, and photos associated with the original activity.
If you recorded with the Strava mobile app and the activity's year is in the 2000s, you may be experiencing an issue related to a GPS rollover that occurred on April 6, 2019. Learn more here.
Check the Strava website to view the activity's total elapsed time. If the total elapsed time is very large, the GPS device has recorded data on different days.
If two activities are recorded together, use the split activity tool.
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.
If you're recording with the Strava Android app, make sure your phone's clock is set to update automatically in your phone's settings.