Get date and time by position of the sun and the observer position I am working on an astrology project (don't panic, my question is about astronomy or math), where I need to figure out some values by the longnitude of the planets.

I have a program for this, the Swiss ephemeris.

With this, I can get the position of the planets by the time and the geolocation of the observer.

What I need to do is to do it backward, so I need to get the exact date and time by the given position of the sun and the position of the observer.

For example, today (2015.12.16 00:00:00 UT) the longitude of the sun is 263.6893755 from postion (lng,lat,alt) 19.2,47.29,0 on Earth.

I need a formula, what tells me, what was the UT time from this longitude of sun -88 degree.

Can anybody give me a clue, how can I calculate this?

I made some research of course on the net, and I found the related wiki article, but if I am honest, there are too many unknown terms and phrase here, what I really do not understand, and I do not want to dig in too deep.
I think you can very easily use the transformations available in the wiki link to obtain the Julian day, then exploit the position of observer to get local sidereal time.
Lets start by computing the Julian day
you have a given longitude $L$ which gives you $n$ number of days since Greenwich noon as
$$n = \dfrac{L-280.4860}{0.9856474}$$
giving the corresponding Julian day as
$$JD = n+2451545$$
once you have Julian day you can use the following link to obtain UT
http://ssd.jpl.nasa.gov/tc.cgi