algorithms related to calculating equinoxes and solstices

Greetings all,

In Chapter.26 of Jean Meeus's Astronomical Algorithms, he introduced a
way to calculate the JDE of the equinoxes and solstices of a given
year. And we all know the apparent longitude of the Sun corresponding
to these calculated JDEs would be 0, 90, 180 and 270 degrees,
respectively.
My question is how can I calculate the JDEs corresponding to other
apparent longitudes, say, 15, 30, 45, 60 degrees, etc. Any idea would
be helpful, thanks in advance.

Cheers,
Difei