Thanks once again to the efforts of Dr. N. C. Ramanujachary, we now have the data from the 1879-1880 Tiru Ganita Panchanga. This allows us to correct two typographical errors in the data given from the 1880-1881 issue (16645009981 for 1664500981, and 1972948980 for 1972948981), which in turn allowed us to correct a typographical error in the data given from the 1884-1885 issue (1955884687 for 1955884987) as copied by H. P. Blavatsky in The Secret Doctrine. All this data from three different issues enables us to eliminate the typographical error factor: the eighteen million years figure is not a mistake. Moreover, since this figure increases rather than decreases, it must in fact be the elapsed years of the Vaivasvata manvantara as stated, rather than the years remaining of the Vaivasvata manvantara lacking one digit, as can be calculated from the extant Sūrya-siddhānta. Since the eighteen million years figure cannot be derived in any known manner from the data given in the extant Sūrya-siddhānta, we are left with only one conclusion. The compiler of the Tiru Ganita Panchanga used a manuscript of the Sūrya-siddhānta that had additional verses in chapter one giving the data necessary to make this calculation.

At the time the Tiru Ganita Panchanga was first published, 1869-1870, there was only one printed edition of the Sūrya-siddhānta. It was published in 1859 in Calcutta (see May 15 posting), far from Madras. Astronomers in India then routinely used their own manuscript copies of texts such as the Sūrya-siddhānta. We must therefore assume that a fuller manuscript of the Sūrya-siddhānta exists in south India. Perhaps it still remains with the descendants of the compiler of the Tiru Ganita Panchanga.

Here is the data from the 1879-1880 Tiru Ganita Panchanga, kindly supplied by Dr. Ramanujachary in his email reply to me dated Aug. 29, 2012:

I had occasion to visit the Adyar Library on this errand today.
Obtained a copy of the first page giving figures.
Yes, your two corrections of years are validated.
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The translated version of the page is as below:

Almanac for the year PRAMADI,
corresponding to Kaliyuga year 4981; and English year 1879-80
The figures are in accordance with SURYASIDDHANTA.
years that passed  in Brahmanah kalpa: 1972948980
years that passed after Srishti(Manifestation); 1955884980
years that passed after SWAYAMBHAVU: 1664500980
years that pased in VAIVASWATA : 18618720

Kali age: 4981
Vikrama Satabda: 1937
Salivahanasak: 1802
Kollamandu: 1054-55
English: 1879-80
Hijari: 1296-97
Phasali: 1288-89
Drik Ganita year: 11