Changing the starting time of a Strava activity sometimes comes in handy. If your colleagues or even your boss follows you, you might want to .... let's say be a bit creative :) Here's how.
1. Upload the file to Strava as you always do, change the privacy to "only me"
2. Go to Strava using a computer (not the app).
3. Find your activity, find the three dots and click on 'Export GPX'
4. Browse to this utility.
5. Upload the GPX, check "time shift" and click on 'upload'.
6. Go to the download page and click on the blue button at the bottom.
7. Delete the original file from your feed.
8. Upload the file manually to Strava