~~~
SQLite version 3.36.0 2021-06-18 18:36:39
sqlite> select julianday('2021-07-20 15:18:26.623');
2459416.13780814
sqlite> select julianday(substr('2021-07-20 15:18:26:623',1,19)||'.'||substr('2021-07-20 15:18:26:623',21,23));
2459416.13780814
sqlite> select julianday(substr(dt,1,19)||'.'||substr(dt,21,23)) from (select '2021-07-20 15:18:26:623' as dt);
2459416.13780814
sqlite>
~~~