The year is given as a complete number in Portuguese.  So for example, 1999 is given as 'one thousand nine hundred and ninety nine' rather than 'nineteen ninety nine'.  The day of the year is given as the cardinal number rather than ordinal, (for example, they say the equivalent of 'day 4' rather than 'the fourth') – except for the first, which is normally ordinal ('primeiro').

Going back further into history, you may need to refer to dates as 'AEC' or 'EC' – that is 'antes era comum' (before our common era), or 'era comum' (of our common era).  You will probably also come across the less accurate but more common 'AC' (antes [de] cristo) which means 'BC' or 'Before Christ', and 'DC' (depois [de] cristo] – which means 'AD' (anno domini – the year of our lord).  Whereas in English we tend to just say the letters (eg. 'B.C.E.'), in Portuguese you have to say the full words (eg. 'antes era comum').  Also, the implied words 'de' and 'nossa' ('of' and 'our') are usually omitted, or sometimes contracted to 'da'.