Gunaakar ia said to be Kalakaachaarya's Guru. In that case there must have been more than one Gunakar or more than one Kaalakaachaarya. Because in the end of Horaa Makarand the author Gunaakar states that -
श्री ब्रह्मगुप्तार्य भटोल्पलाख्य वराह लल्लेषु दिवं गतेषु।
निराश्रयेयं समवेक्ष्य वाणि विलासवासां रचयां चकार।।
From Horaa Makarand
Means that - Horaa Makrand by Daivagya Gunaakar got written after the period of scholars like Brahmagupt, Aaryabhaat, (Bhatta) Ulpala, Varaah Mihir and Lalla.

Since the period of Bhattolpal is considered to be 10th century AD, this will mean that 'Horaa Makarand' got written by 'Daivagya Gunaakar' after 10th century AD only. No wonder that Horaa Makarand is like an imitation of Brihat Jaatak.

Times of Kalakacharya (Veer 599-527 BC),
Kumaaril Bhatt (557-493 BC) and
Aadi Shankar (509-476 BC) have been given in Jinavijaya Mahaakaavya written in praise of Jain religion, but all Indian Jains believe in Oxford texts and not in any Jain text.
Period of all Magadh kings have been given in various Puraan and that remains only source of list of kings and their ruling periods.

// Mihira was born in the beginning of the last quarter of the 2nd century BCE.//
If so, how come Panch Saiddhaantikaa speaks of years such as AD 505 or 525 or so? Further what about Reference to Aaryabhatt's theories based on the era AD 498 or so?

Varaah Mihir and Brahmgupt were contemporaries of Vikramaaditya of Ujjain (82 BC - 19 AD)
So Varaah Mihir was in 1st century BC (Kalhan gives the dates 505-587 AD) on Raaj Taranginee p 4.
Brahmgupt was in 1st century BC
Shaalivaahan was 2nd century AD - he defeated Vikramaaditya

Bhaaskaraachaarya (1114-1185 AD)
=========================================================

 It is obvious that Varahamihir could not use any era which started 90 years after his death. He has never given dates of 505 or 525 AD - it is deliberate wrong calculation of dates from Shaalivaahan shak.

There is another deliberate falsehood about Shak Nrip Kaal. Yudhishthir Shak started 36 years before Kali - Yudhishthir was not of Shak tribe. Siddhaarth, son of Shuddhodan was called Shaakya Muni - he too was not of Shak. Shak trees in north and south India are Sal (Sakhua) and Teak (Saagvaan) - these were not brought by Shak invaders nor they resided here.

No Shak king has ever started any calendar in his own country of > central Asia or south east Europe - how it could be used in India. They followed Sumerian calendar. There was no running calendar in Persia itself, how it could be followed in India? Nothing can be explained to a person who does not want to know.

Every Calender has always mathematical and civil aspects. It was called Amali & Fasali by Akbar who is said to be illiterate. But educated
astronomers do not want to know. Almost in same sense, Shak and Samvatsar are used. that too is obvious if one sees any text book.

All calculation manuals are based now on Shaalivaahan Shak. All decisions on festivals are based on Vikram Samvat - Dharm Sindhu, Nirnaya Sindhu, Vidhaan Paarijaat etc. Any king who started a running calendar was called Shaka Kartaa, or Shak Nrip.
=======================================================

Books of Vikramaaditya period-
(1) All Puraan - Puraan were updated under the guidance of Vetaala Bhatt - one of the 9 gems of his court. The places where this was arranged were called Vishaalaa, like Mahaashaalaa of Shaunak. There are 3 places known as Vishaalaa, one is Ujjayinee itself, capital of Vikramaaditya (Meghadoot 1/32 of Kaalidaas), Vaishaalee in Bihaar (famous as place of Bhagvaan Mahaaveer), and Badaree Vishaal, where Aadi Shankar had written his Brahm Sootra Bhaashya in 494 BC. There could be a fourth in South India also.
(2) Hariswaamee commentary on Shatapath Braahman - at the end of the each chapter, Hariswaamee has called himself as Dharmaadhyaksh under Vikramaaditya.
(3) Many Smriti texts.
(4) Tantra texts by Ghatakharpar - no earlier texts are available on Tantra.
(5) Jain Aagam by Kshapanak (= Shraman or Jain Muni).
(6) Sushrut Sanhitaa by Dhanvantari (3rd), his one of the Nava Ratn
(7) Epics of Kaalidaas (2nd) indicated in Jyotirvid Aabharan.
(8) Astronomy astrology texts of Varaah Mihir - Brihat Sanhitaa, Brihat Jaatak, Panch Saiddhaantikaa, of Brahmgupt - Braahm Sphoot -Siddhaant, Khand Khaadyak. Many others by Vararuchi (Vaakya Kaaran), Satyaachaarya, Manittha etc.
(9) Revenue records by Shanku - revived by Akbar who copied Nava Ratn system also. Shanku is the instrument used for surveying described in Soorya Siddhaant and other texts on astronomy.
(10) Amar Kosh by Amar Sinha, grammar by Vararuchi etc.

There are many others. Main feature was that till the time of Vikramaaditya, Sanskrit was standard language. From time of his grandson Shaalivaahan, literature in local languages started which flourished under Gorakhnaath in 800 AD. (Saraswatee Kanthaabharan Alankaar of Bhoj, verse 32).
==============================================

Major kings of 4 Agni Vanshee from Vikramaaditya to Prithvee Raaj Chauhaan and last Bhoj of Maalavaa were ignored.

(7) Shreeharsh (456 BC) was the first king to assume the title of Vikramaaditya. His elder brother Bhartrihari became Sanyaasee and wrote 3 famous Shatak (Neeti, Shringaar, Vairaagya) and Vaakyapadeeyam (philosophy of grammar). Raaj Taranginee indicates that he is confused with Paramaar king Vikramaaditya whose name itself was that, not a title. Shreeharsh had deputed Maatrigupt (Title Kaalidaas) for 5 years to rule Kashmeer in absence of any king there.