Some people talk of 100 years of Delhi as Indiaâ€™s capital. Delhi has been Indiaâ€™s capital at different times since 1206 when Delhi Sultanate was established. After that the sultans ruled from different parts of Delhi. Allauddin Khilji built and ruled from Siri. Ghiyasuddin Tughlak built Tughlakabad. Mohammed Bin Tughlak built Jahanpanah. For some time he ruled from Daulatabad. Feroz Shah Tughlak built Ferozabad. In 1526 Babur founded the Mughal Empire. Humayun built Din Panah. Sher Shah Suri razed it and built Sher Shahi. Akbar made Agra the capital of Mughal Empire after ruling from Fatehpur Sikri from some years. Shah Jahan shifted the capital to Delhi. He built Shahjahanabad. After Aurangzebâ€™s death the Mughal Empire declined.
Calcutta was the capital of the British Empire in India. Simla was Indiaâ€™s summer capital from 1865 to 1939. On 12/12/1911 at Delhi Durbar George V announced the shifting of capital from Calcutta to Delhi. Notification was issued in January 1912 and construction of New Delhi began after that and was completed in 1931. In case of New Delhi it is 80 years as Indiaâ€™s capital.
The British Empire in India once included Mauritius, Singapore, Penang and parts of Ceylon. When Calcutta was the capital of India the British Empire was at its zenith. Aden was separated from India in 1932. Burma was separated from India in 1937. In 1947 Pakistan was separated from India and British rule came to an end in India. The British built New Delhi to rule India for ever. They wanted to rival the Mughals in splendour. They built a large Viceroyâ€™s House which is now Rashtrapati Bhavan. Within 16 years of change of capital it was the end of the empire in India. The Statute of Westminster passed by British Parliament in 1931 made Canada, Australia, New Zealand and South Africa independent countries. Thus 1931 was a turning point in the history of India and British Empire.
We can rightly say Delhi as capital of India since 1206 except for some breaks or for 80 years. Announcement of shifting of capital to Delhi does not make it capital on the same day.