In FSX AI flight plans what does day 1 correspond to? Sunday or Monday?

From the SDK:

 

departureTime = DHHMM, where D = optional day of the week (0=Monday, 1=Tuesday, etc) and HHMM = zulu hour and minute of the day

So FS9 has 0=Sunday, that's the difference?

I think so. From memory (it's been a few years since I've done a traffic file) there was a change of the days numbering order from FS9 to FSX. 0=sunday would be logically correct (I can't imagine it would be 0=tuesday).

 

A bit of google research shows this from the AIFPC program support page ( http://aifs2.pvdveen.net/?cat=4 ) which confirms it:

 

"Another difference between FS9 and FSX is the numbering of the days. In FS9 the week starts on Sunday which has the number 0. In FSX the week starts on Monday with the number 0. So when using FS9 compiled flight plans there is a shift in the days. Flights who would departure on Sunday in FS9 will departure on Monday in FSX."