Can't believe this feature hasn't been implemented. I have had a bunch of issues trying to find an app that I can simply insert the date and time and then be able to write comments next to it. I just found this app and it seems to work really well. I have it set up in the app tray so I just swipe down, click the button, and then it automatically opens w/e app I want. Then I just paste the copied timestamp.
There's a free demo version which adds a second or two before continuing to the program after you copy and paste. The paid app is a bit over a dollar. I just used my google rewards to pay for it. Anyways, try it out if you'd like, it's exactly what I needed.
Full App: https://play.google.com/store/apps/details?id=shhh.Apps.TimeShare
Demo App: https://play.google.com/store/apps/details?id=shhh.Apps.TimeShareTrial