strdate("%y%m%d %H:%M:%S", wdate()) doesn't show second's decimals, did I miss something in format? %S should show optional decimals but how to exercise this option?

%S does not display decimals. There is no format code for displaying decimals. Only when you import a date, %S gets also the decimals because that is a Zorro function, not a standard Windows function.

For decimals, round down the date to the recent second, subtract, and display the difference as decimals.