When we tested this issue on our app My Day a month ago the widget didn’t show up in the widget list, but when we tried doing that again an hour ago, it actually did show up.
To that I can only add that widgets are rather unpredictable per se.
I have two i OS devices: i Phone 6s and i Pad 3rd gen, both updated to the most current available i OS. On the i Pad, however, the widget's updates are sporadic, at best.
It is normally a couple of days behind, or at least until last month.
My Day and it helps you keep track of the most important days in your life.