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I have been writing my own ephemeride program for my own interest. However, I encounter a problem when I use it to generate Chinese lunisolar calendar. While the program succeeds to produce the Chinese calendar for year 1929-2043 correctly (verified against official ones), it fails for year 1979. In particular, what's the exact time of Dahan [en.wikipedia.org] (大寒; major cold) in year 1979?

By definition, the major cold begins when the apparent longitude of the sun (with respect to the mean equinox) reaches 300°. (Is this definition correct?) To find out the exact answer, we can refer to some accurate ephemeride such as the JPL Horizons On-Line Ephemeris System that makes use of DE405/DE406/DE413, which are generally considered the most accurate ephemerides to date.

Examine the apparent longitude of the sun of every minute (in Terrestrial Time) on 20th Jan 1979, we can find that:

At 1979-Jan-20, 16:00 TT, the longitude was 299.9994615°;
At 1979-Jan-20, 16:01 TT, the longitude was 300.0001683°.

By doing a parabolic interpolation with five tabular values, we can find that the longitude turned 300° at 1979-Jan-20, 16:00:45.712 TT (Terrestrial Time). At that time, the value of Delta-T [en.wikipedia.org] (= UT - TT) is 50.184s. Thus, the event happened at 1979-Jan-20, 15:59:55.528 UT (Universal Time). Note that the absolute difference of UT and UTC (Coordinated Universal Time) is no more than 0.9s. Therefore, in Beijing of China, which is within the UTC+8 time zone, the event happened no later than 1979-Jan-20, 23:59:57.

However, from many resources on Chinese lunisolar calendar I have checked, the Dahan for year 1979 fell on Jan-21 00:00 --- less than one minute but also one day later than the calculation above! Why is there such a discrepancy? More findings about this issue:

If you use the JPL Horizons system to check the apparent longitude of the sun at 1979-Jan-21 00:00:00 UT, it shows that the longitude is at 300.0000526°, or 300° 0' 0".1894. So it means that Dahan should happened before that time.
From the previous point, to calculate this Dahan accurately, an algorithm or an ephemeride with an accuracy better than 0".1894 is required. Was such an ephemeride available that time? Nonetheless, observatory equipments with such an accuracy should be available that time.
The Chinese lunisolar calendar had been 'wrongly' calculated especially before year 1929, due to the lack of accurate ephemeride. But this mismatch happened in year 1979, which is considered quite recent. Why?
I found that an open-source Chinese lunisolar calendar program 'correctly' (or incorrectly?) calculated that the Dahan fell on Jan-21. Further examination shows that the program misused the Terrestrial Time instead of the Universal Time to perform the calculation. So I suppose that the result is not correct. We need to use UT instead of TT, right?

I have not found an explanation to the discrepancy. I need it to solve the problem in my own ephemeride program. Is there anything wrong in my calculation? Tell me if you know, thanks!