If you got the data in excel, you can use this formula: =A1/86400000+(-7/24)+DATE(1970,1,1) where A1 is the cell the date is in and the -7 in the (-7/24) is the timezone offset in hours. You may need to tell Excel to format as date and time.
The trend timestamps are in UNIX epoch format * 1000 (for milliseconds).
UNIX epoch format is seconds since 1/1/1970
Excel stores dates as a floating point as days since 1/1/1900. The fractional part is the portion of the day.
The formula above converts from the former to the latter.