"Mil naw saith dim"
As already explained this is a special format for a date, literally Thousand nine seven zero (1970)
It's a very simple pattern, for all dates from 1000 to 1999 it's just mil followed by each following digit expressed as a simple number.
The new millennium starts with Dwy fill followed by the digits.
eg 2016 = dwy fil un chwech or sometimes dwy fil un deg chwech
For the numbers after a thousand then eg 1970 = un mil naw cant saith deg or un mil naw cant a saith deg etc.
There are different possible answers here:- eg The sentence:- 'In the year six hundred and seventeen' would sound better to me as 'Yn y flwyddyn chwech un saith but the other suggestion 'Chwe chant un saith' and also 'Chwe chant un deg saith' are also forms I've heard.
43 = Forty Three AD (or 43 CE as it often called these days) is usually Pedwar deg tri OC (OC = Ar *Ôl C**rist)