This is my set of AutoHotKey macros which insert timestamps in various formats not only in N++ but across the entire OS. You can adjust the format string as you need.
#F2:: ;Date/time with safe characters for use in filenames
FormatTime, TimeString,, yyyyy-MM-dd_HH-mm-ss
Send %TimeString%
Return
#F3:: ;Short date
FormatTime, TimeString,, yyyyy-MM-dd
Send %TimeString%
Return
#F4:: ;Shortest possible date
FormatTime, TimeString,, yyMMdd
Send %TimeString%
Return
#F5:: ;Date/time
FormatTime, TimeString,, yyyyy-MM-dd HH:mm:ss
Send %TimeString%
Return
So this way Win+F5 inserts timestamp containing date and time.
You can limit the scope only to Notepad++ by using #IfWinActive directive on row above the macros. I am using this one:
#IfWinActive ahk_class Notepad\+\+