I pronounce it day-ta because dah-ta to me sounds like a baby trying to say daddy, especially if you don't pronounce the hard T. For some reason day-ta sounds more professional to me but also I'm not a native speaker so maybe my view is different from most native speakers in that regard.
Also, the singular of data is datum. Do you pronounce it day-tum or dah-tum? Because that I think influences how you should then pronounce data.
Also, the singular of data is datum. Do you pronounce it day-tum or dah-tum? Because that I think influences how you should then pronounce data.