What format is the time of a Bitcoin transaction stored in?
If you are literally referring to the "time" or "blocktime" property of a transaction within the blockchain, then this timestamp is in Unix format.

And if by "convert it to a meaningful value", you mean a human-readable format, you can use a unix command line (e.g. Terminal on OSX) to do a quick conversion:

Linux:

$ date -d @1395103695


Mac OSX:

$ date -j -f "%s" 1395103695


...which would return the following:

Mon Mar 17 14:48:15 HST 2014