If, upon attempting to upload a GPS file, you see an error message telling you that "Time information is missing from the file," this means that your file is incompatible for upload to Strava.
What does this error mean?
The problem is that your file likely contains only GPS data and no timestamp associated with each latitude and longitude coordinate. Opening your file in a text editor might reveal something like this:
<trkpt lat="37.7808230" lon="-122.3956800">
Strava can't calculate your speed and time on segments without timestamps, so the files will be rejected upon upload.
Common sources for files like this
If you are a Strava subscriber, you have the ability to download the .GPX files from other users' activities. However, these files will not contain time information. If you are trying to 'copy' an activity from another Strava user with whom you ran or rode, you can do so by asking them to export and send you their own .GPX file; when they do so that file will contain timestamps.
Another common source for these files is .GPX files are exported directly from MapMyFitness sites. These files do not contain workout data, which includes timestamps.
If you can not obtain GPS files with time information you can always create a manual activity on Strava. As always, if you have any additional questions please proceed with submitting a support ticket.