LinkedIn's API returns the following value:
[creationTimestamp] => 1407247548000
It looks similar to a UNIX timestamp, but there are three "extra" zeros at the end. What format is this in, and how can I decode it?
It is a timestamp in milliseconds. Handling this is language dependent. Some languages may expect a timestamp in milliseconds, while others may expect it in seconds. Python 3, for example, expects seconds, but also handles microseconds (1000 milliseconds).
from datetime import datetime
ts = 1407247548124
dt = datetime.utcfromtimestamp(ts / 1000)
print(dt) # datetime(2014, 8, 5, 14, 5, 48, 124000)
Python 2 doesn't handle milliseconds directly (it ignores the fractional part), so you need to split the milliseconds out separately.
from datetime import datetime
ts = 14072475481234
secs, millis = divmod(ts, 1000)
dt = datetime.utcfromtimestamp(secs).replace(microsecond=millis * 1000)
print(dt) # datetime(2014, 8, 5, 14, 5, 48, 124000)