Today we talk about timestamps in http header values. Caching mechanisms for example are based on header fields that carry absolute timestamp values. We review the possible errors in using this field from an old WRL research report.
First of this is date skew. When compared with the NTP server with an accuracy up to milliseconds, the date headers often show that the clocks from the originating servers are not synchronized.
Second is impossible last modified value. The last-modified response header gives the timestamp at which the resource was most recently modified. A cache can validate a stored response by asking the server using the if-modified-since request header. This value therefore should not be any newer than the response's Date value but many often are not.
Third is the apparent coherency failures in using Last-Modified. The use of this field implies that the server responding to the URL is caching  and that the contents of the URL has not changed. When this is not the case, we have a coherency failure. Many of the servers seem to demonstrate this anomaly.
Fourth is the effect of the limited timestamp resolution. The Last modified header has a resolution of 1 second. Therefore even a correctly implemented server that changes its content more than once in a second may have inaccurate timestamps.
Fifth is the effect of apparent premature expirations. An origin server may assign an expiration time after which the cache has to revalidate a stored response. Servers usually assign expiration times heuristically, guessing how long it might be before the resource is changed. By comparing the maximum age value, one can tell whether a cached response expires before the response value changes.
Sixth is the over optimistic expirations. When we compared a conservative expiration time effect above, we could also complement a similar caveat with generous expiration times that may cause incorrect caching.
To summarize, server operators may need to keep their clocks adjusted and Caches have to be consistent and correct in their use of timestamps.

#codingexercise
print time in RFC 1123 format
from email.utils import formatdate
print formatdate(timeval=None, localtime=False, usegmt=True)
Sun, 30 Nov 2014 16:43:49 GMT

generate random alphanumeric
import random
print ''.join([random.choice(string.digits + string.ascii_uppercase) for i in range (0,12)])
D8KCQ5WT6TLN

How do you package for debian?

cd apisecurity-1.0
dh_make --native
echo "data/* usr/share/apisecurity/" > debian/apisecurity.install 
# edit rules, menu, copyright, changelog

gpg --list-secret-keys

dpkg-buildpackage -us -uc

debsign -k 1FB5E9A2 apisecurity_1.0_amd64.changes

dupload apisecurity_1.0_amd64.changes

#codingexercise
Int[] GetDistinctRange(int [] A)
{
if (A == null) return null;
return A.DistinctRange();
}