Convert milliseconds to date (in Excel)
Converting your value in milliseconds to days is simply (MsValue / 86,400,000)
We can get 1/1/1970 as numeric value by DATE(1970,1,1)
= (MsValueCellReference / 86400000) + DATE(1970,1,1)
Using your value of 1271664970687 and formatting it as dd/mm/yyyy hh:mm:ss gives me a date and time of 19/04/2010 08:16:11
See Converting unix timestamp to excel date-time forum thread.