does have a facility within it’s DP converter page that provides BJD time either on board or tick a box and it supplies the time from the uni of Ohio site mentioned on other earlier reply. Some useful instructions and manual provided by Denis Conti at;
The GPL Positional Astronomy Library (PAL) can be found on Github here. It is an open-source reworking of Patrick Wallace’s slalib and it contains all the functions you need to do this. Alternatively the original slalib in Fortran is GPL and should be available somewhere. Even if you can’t use these functions directly they are a good source of information on how to do the conversion accurately.