I convert my RAW (NEF) files to DNG when I import them to Lightroom 5. I've just noticed that since 31st July, in Windows Explorer, the "File Created" date on the DNG file is not the same as the date it was actually converted from NEF to DNG. For instance, I took a photo this morning (8th August) & imported it into Lightroom, converting it to a DNG on the way. When I look at the file created date in the folder in Windows (7), it says 31st July. Odd. The metadata is correct, though, as is the system date & time on my PC. I've done a bit more checking & it started to happen after I updated Lightroom to v5.6.
If you download and install the Adobe DNG Coded (version 2.0) the correct date will then show in Windows Explorer.