The asteroid is probably bright enough to record with a DSLR video through a scope with a bit of aperture. The flash tag apps Dave referred to are both free and simple to use.
Two versions are available -
Astro Flash Timer for iPhone (free)
AstroFlashTimer provides precise timestamp flashes based on GPS timing from the camera LED for astronomical videos (primarily asteroidal occultation recordings). Just point the camera flash into the optical path of the video recording device and trigger the desired flash mode. The app will log the exact time of the video flash onto your iPhone. The timing information can then be sent from the iPhone by email or text for later analysis. It is available from the App Store or from here : -
https://sites.google.com/site/astroflashtimer/home
Occult Flash Tag for Android (free)
Occult Flash Tag provides precise timestamp flashes from the camera LED for astronomical videos (primarily asteroidal occultation recordings). Just point the camera flash into the optical path of the video recording device and trigger the desired flash mode. The app will log the exact time of the video flash onto your Android device. The timing information can then be sent from the Android phone by email or text for later analysis. It is available on Google Play -
https://tinyurl.com/y8otw47b
So there is no reason not to have a time stamp on your video.
Joe