It used to be pretty common to see the ole time and date stamp on the corner of photos back in the day. However, with new technology it has phased in the background. This has left private investigators utilizing other ways of trying to put a time date stamp on their photos. I've came across two apps that I use on my android device that easily lets me put a good time stamp on my photos and another for videos. For the camcorder I use this software and it has been worked extremely well for me. Link to DVMP Pro 6
However, I don't always use my handy camcorder as I generally have my phone on me more frequently. For photo's I use Camera Auto Time Stamp (I have no affiliation, but it is very dependable and runs in the background).
Right now I use an LG G4 with an obscene Zerolemon extended battery on it and a 128gb sd card ready to go. The battery is 3.5x the size of a normal battery and does make it fat and hefty, but considering the amount of use I can pull from a smartphone its worth it. Given the camera has a hefty 8x zoom and high quality, I can gather quite a bit of surveillance from a tiny little platform.
For videos I use the Video Timestamp Add On. Again, I have no affiliation but I've had great success using both of these apps and have found them to be dependable and not resource heavy.
Why is having a time date stamp important? Well, to prove your evidence in court of course or to catch someone in a lie.