"The most fundamental unit of time-measurement is in most societies the period of the earth’s rotation round the sun, which is normally known as the day. Unfortunately this word and its equivalents in other languages are ambiguous: other meaningsapart, they may denote either the light period (daytime) as opposed to the night, or the combination of daytime and night. In some cultures, this combination is termed the night, as it used to be by Celtic and Germanic peoples, who measured the length of journeys or campaigns by the periods of inaction during darkness; this practice – to which we still revert when booking a hotel – survives in the English word ‘fortnight’, meaning 14 nights (formerly too in ‘sennight’, meaning a week). Nevertheless, the prevailing word is ‘day’."
Physics Leofranc Holford-Strevens's The History of Time: A Very Short Introduction has been given a preview chapter at the OUP's website in pdf format: