In the modern period, Pt. V.N. Bhatkhande popularised the Time theory based upon 24 hours. He divided 24 hours into two parts. The first part commences from morning 12 to night 12 and second part from night 12 to morning 12 The first part also known as 'Purva Bhag' and Purvang Vadi Ragas are performed during this time. The second part also known as 'Uttar Bhag' and Uttarang Vadi Ragas are performed during this period. This Purvang vadi ragas have their vadi swara in the Purvang, the Lower Tetrachord and Uttarang Vadi raga have their vadi swara in Uttarang, the upper tetrachord.
He also created, three categories based upon, 'swaras' and 'time'. The first category is of 'Re-Dha Komal also known as Sandhi Prakash ragas, the second category is of 'Re-Dha Shuddha' and the third category is of 'Ga-Ni Komal'. Shuddha Madhyam also plays an important role. Morning Sandhi Prakash ragas have the usage of Shuddha madhyam and evening Sandhi Prakash ragas use Teevra Madhyam. Madhyam is also known as Adhwadarshak Swara. In the vast area of Ragas, fitting all into time frame under these categories is a scholarly task.