The timestamps in Call Detail Record (CDR) are in UNIX time, which is the number of seconds since 1/1/1970 (midnight GMT).
For example, if you have a timestamp in an Excel spreadsheet in cell B1, you could use a formula like :DATE(1970,1,1)+B1/(60*60*24) to return the date portion, and a formula like :TIME(24-8,0,0)+B1/(60*60*24) to return the time portion. The "24-8" in the time function converts the tim ezone to GMT-8 (PST).
If you know the timestamp for a particular date, you can calulate the timestamp for another date by adding or subtracting 86400 (number of seconds in a day), multiplied by the number of days to get to the date you want. In Excel, if you have a date in cell A1 and a time in cell B1, you can use a formula like this:
to return the UNIX timestamp. The 24-8 in the Time function converts the time zone to GMT-8 (PST).