SQLite Forum

Convert datetime string to second since Epoch with millisecond precision
This would return a volatile time.  The builtin datetime functions return statement stable datetime values.  

If you write a function that calls the VFS method to get the "CurrentTime" you will get the "CurrentTime", that is the exact nanosecond as which the call was made.

If you use one of the builtin datetime functions, then the datetime will remain constant throughout execution of that statement.