Dear stata community

I have a data-set that is combined from two different datasets, by append. I have a variable for time and date in each of the original datasets, and now I have two variables with time and date. How can I combine these (they have the same format)? And then I want to make a new variable that shows only the year, so I can sort observations within a patientID, on year.

Code:
* Example generated by -dataex-. For more info, type help dataex
clear
input str9 PatientID float(dato2 date2)
"1" 19981     .
"1" 20394     .
"2" 20922     .
"2"     . 20149
"2"     . 19308
"3"     . 20579
"3" 19756     .
"4" 20579     .
"4"     . 19471
"4" 19797     .
end
format %tdCCYY-NN-DD dato2
format %tdCCYY-NN-DD date2

Cathrine