Merritt Gardner Kellogg (28 March 1832 – December 20 1921) was a Seventh-day Adventist (SDA) carpenter, missionary, pastor and doctor who worked in Northern California, the South Pacific, and Australia.

Kellogg was involved in the controversy about which day should be observed as the Sabbath on Tonga, which lies east of the 180° meridian but west of the International Date Line.

Merritt Gardner Kellogg was born in Hadley, Massachusetts on the Connecticut River on 28 March 1832.

In 1861 Kellogg gave a series of Bible lectures in San Francisco in which he converted fourteen people.

[2] During his return journey to California he attended the SDA General Conference session on 1868 and asked that the church send evangelists to the West Coast.

[4] Back in California he assisted the evangelists John Loughborough and Daniel Bourdeau, mostly talking on topics related to health.

[2] During a smallpox epidemic in 1870 he gave water treatments and diet to his patients, of whom ten out of eleven survived.

[5] In the summer of 1877 Kellogg was asked to help look after patients at a San Francisco hydrotherapy center in exchange for room and board.

The owner, Barlow J. Smith, sold out after five months and opened a retreat in Rutherford, in the Napa Valley.

[7] Kellogg stayed in Australia for a while, where he made a second marriage to a woman from Broken Hill, New South Wales.

[11] Kellogg and fellow-missionaries Edwin Butz and Edward Hilliard debated which was the correct day to observe the Sabbath in Tonga.

Islands such as Samoa and Tonga were well to the east of this line, so the missionaries observed the Sabbath on the day sequence of the Western Hemisphere.

[2] The tract by Adventist John N. Andrews called The Definitive Seventh Day (1871) recommended using a Bering Strait date line.