When did Eratosthenes calculate the circumference of the earth?

Eratosthenes lived from ~ 276 BC - c. 195 BC. In the text In On the Circular Motions of the Celestial Bodies, Cleomedes credited him with having calculated the Earth's circumference around 240 BC, which would have made him around 36 years old at the time. For his part, Cleomedes wrote his text between between the mid-1st century BC and 400 AD (although probably between about 100 BC and 100 AD), so he was writing about a measurement that occurred hundreds of years in the past so we can only consider the 240 BC date as a reasonable estimate rather than established fact.