Moses Bruine Cotsworth was born on December 3, 1859, in Willitoft, near York, England. He was the son of George and Sarah Cotsworth. Orphaned at the age of two, he was raised by his grandparents and great-grandparents. In 1884, he married Kezia H. Gardiner, (of Blairendinnie, Aberdeenshire), and they had one son and four daughters. He was educated at York Blue Coat School.
Cotsworth began work in 1874 in the office of the Chief Goods Manager of the North Eastern Railway Company, York. He had a series of jobs in which his aptitude for calculations and his investigative skill were evident: in 1881, he worked at Aire and Calder Navigation; from 1884 to 1886, he was the traffic manager for Trent Navigation; and from 1887 to 1891, he was the manager of Manners Colliery, Derbyshire. In 1892, he returned to the Chief Goods Manager office to work on a revision of railway rates and to do research. He published a book, Railway Maximum Rates and Charges, and his improved system of railway statistics has since been used by the British Railway companies and the Ministry of Transport. In 1907, he was appointed by the British Columbia government as Chairman of the Commission to re-organise and regrade the Civil Service. He moved to Vancouver in 1910, and from then until 1921, he was in business as an accountant and investigator of the cost of living for Western Canada.
Cotsworth devoted most of his time and efforts on calendar reform. His interest in calendars and time goes back to his upbringing by his grandparents and great-grandparents who used the old shadow pin, noon-mark, and hour-glass methods as well as an old series of calendars which contained charts, illustrations, and quotations. While he was working for the North Eastern Railway Company, he became aware of the difficulties in using the Gregorian calendar to calculate monthly totals for income and expenditures. These difficulties were due to the different number of weekdays in a month and the fact that, for the railway, every day of the week had a different economic value. The yearly changes of the date for Easter, and other holidays, imposed another difficulty. Even before he founded the International Fixed Calendar League in 1922, he was active in calendar reform. Cotsworth devised a thirteen-month calendar in which each month would have twenty-eight days or four weeks. The thirteenth month, called Sol, would be made up of the last thirteen days of June and the first fifteen days of July. In leap years, the extra day would be added after June the 28th. The 365th day, at the end of the year, would be called Year-day, and it would fall between December the 28th and January the 1st. In this way, Easter would always fall on the same date (the 15th of April). At one point, both the League of Nations and the Royal Society of Canada endorsed Cotsworth's proposal for calendar reform. The federal government of Canada officially endorsed it in 1931. However, Cotsworth's 13-month calendar was not adopted due to lack of world-wide consensus and to other individuals or calendar associations pushing for calendar reform.
Even though the International Fixed Calendar League ceased to exist, Cotsworth continued his work on calendar reform. From 1922 to 1931, he was the Director of the International Fixed Calendar League (previously called the International Almanak Reform League), as well as the Expert to the League of Nations' Committee on Calendar Reform. As such, he visited more than sixty countries as part of his effort to convince people of the benefits of converting to a 13-month calendar. He served as Secretary to the British Association's Research Committee on Geological Evidence of Climactic Change from 1934 to 1937. He was a Fellow of the Institute of Chartered Accountants, a Fellow of the Royal Statistical Society, and a Fellow of the Geological Society. His publications include monographs as well as a series of more than 50 pamphlets on calendar reform. He also wrote about railway rates and climactic change, among other topics. Moses Bruine Cotsworth died in Vancouver on June 4, 1943 .