I need to know the exact date of image acquisition for NDVI purpose, the date on image name is " LC80920852014222LGN00" is in Julian date, but in meta data it says"acquisition_date>2014-08-10" , I assume that I can use the date in meta data, any ideas?
Yes, you can use the acquisition_date in the metadata, which is the date in ISO format (YYYY-MM-DD).
The date in the filename is Year+Julian Day.
You can check the acquisition date against the filename if you wish using the NASA Julian Day Calendar
This shows 2014-08-10 = 2014222