The Letitia Gilland i looked at had a estimated birth of 1868
You cannot correctly 'calculate' a year of birth from an age in the census. This is because the census was always (except in 1841) taken around the end of March. Therefore people are three times more likely not to have had a birthday in census year so far, so by the end of the year three out of four people will be a year older. Only one in four people will already have had their birthday in census year, so only one in four 'calculations' will get the right answer.
The one in the 1891 census was listed as 23. If that was accurate, then she was born in 1867 or 1868. If her age in the census is just one year out, which is not unheard of, she could have been born in 1866 or 1867.
The 1881 census transcription I have says that Joseph Gilland, 36, alum work stoker, born Ireland, was living at Whyte's Land, Dunlop Street, Abbey (i.e. Paisley), Renfrewshire with wife Margaret, 37, born Ireland and five children, the eldest being daughter Letitia, cotton mill worker, 14, born Abbey. An age of 14 in the 1881 census is spot on for someone born on 7 November 1866. This family matches the births in the IGI at FamilySearch to Joseph Gilliland and Margaret Wigans. The Irish Civil Registration indexes on FamilySearch list the marriage of Joseph Gilleland at Cootehill in 1865, Volume 18, Page 153, and there is an exactly matching listing for Margaret Wiggins. Cootehill is in County Cavan, in the Republic of Ireland.
William Gillan, aged 42, boiler maker, born Kirkintilloch, was at 4 Spring Lane, Govan in 1881 with wife Jane, 42, born Glasgow and six children including James, 17, rivetter, born Greenock. There is also a nephew, James McVey, 16, iron moulder, born Glasgow. (I speculate that this is the illegitimate son of Jane Gillan, born in Hutchesontown on 25 February 1865.)
William Gillan and Jane Black were married in Hutchesontown on 27 May 1861. Their marriage certificate will tell you the names of William's parents.
