There are five types of years in astrology. 1.solar year(saura mana 365.266 days) 2.lunar year(chandramana 354days ) 3.nakshatra mana(324 days) 4.brahaspatya (jupiterian year 395 days) 5. Savanamana (360 days year)
Each has its own significance in astrology. For mass regulation solar year is best suited. for day today affairs lunar year should be adopted. For shanti nakshatra year is adopted . For political forecast brahaspatya is used, For birthdays,longevity,dasas vrata,niyama,yagna,Savana year is used.
Indian panchanga is soli-lunar calendar. It takes into account 360 tithis rather than days. it is unique and very apt for dharma shastras. This takes into account the tithi at sunrise, and this remains for entire day for the celebration of certain festivals. For example, if at sunrise ,it is Navami , then whole day that day will be considered as RAMNAVAMI , though in actual practise dashami might have come by noon.
For shradhha , whatever tithi is at noon is considered as shrahdhha tithi . So eve if you have dwitiya in the morning , but tritiya at noon then tritiya shradhha can be performed.For vratas , tithi mixed with previous tithi is prohibited, tithi mixed with forward tithi is auspicious. But for sankashta chaturthi , tithi at evening chandrodaya is only apt.Krishnaashtami is celebrated on a day when both rohini or ashtami is bound on midnight , it is more auspicious if it is wednesday on that day. Fasting is done till both ashtami and Rohini are exhausted.
Thus panchanga enables us to perform our daily karmas (Nitya karmas and Naimittika karmas ,and kamya karmas) at a proper time to avail destined results. Nitya karmas are those that are must , like bathing everyday, sandhyavandane ,ekadashi etc.These if not done attracts sins. Naimittika are those which are done on special occassions like Magha snana , Kumbha snana etc. These if done gives merit ,but does not culminate into sin not done. Kamya karma are those which are done to achieve certain goals. All these should be done at right time to get results. Astrology helps to time these with the help of five types of calculation Panchavidha samvatsara.