The order of months in a year does not change, neither does the order of days in a month. January in a particular year is always before December in the same year.
So, for your example, 25 January 500 BCE is earlier than 25 December 500 BCE.
As a more general point, it is worth writing historic dates out in full, rather than using numbers for months.
To illustrate the problem, consider the date
9/11/2001. Now, everyone will probably recognise this as the date of the attacks on the World Trade Center and Pentagon on 11 September 2001. However, without that context, the date
9/11/2001 would be read as 9 November 2001 in the UK and other parts of the Commonwealth.
(Given the magnitude of the events of 9/11, even most UK pedants don't argue the point in this particular instance, although Brits often have to pause to make the adjustment to a US date format).
For clarity, it is always worth writing dates out in full so there can be no confusion.