HTTP If-Modified-Since with milliseconds

Question

Let's say I get an object from REST web service and this object has a time stamp. This time stamp has a milliseconds component. Next time I request the same object I don't want it to be returned unless it has changed, so I use the If-Modified-Since header. But the date in that header is not even supposed to have milliseconds. If I round the time stamp down, I'll always get the object back as if it's always modified. If I round it up I risk missing some updates. Is the If-Modified-Since header completely useless to me in this case, or am I missing something?

Answer 1

A service that sends a time stamp with milliseconds is not HTTP-compliant. Last-Modified MUST be sent as a HTTP-date which §3.3.1 specifies very clearly:

HTTP-date    = rfc1123-date | rfc850-date | asctime-date
rfc1123-date = wkday "," SP date1 SP time SP "GMT"
rfc850-date  = weekday "," SP date2 SP time SP "GMT"
asctime-date = wkday SP date3 SP time SP 4DIGIT
date1        = 2DIGIT SP month SP 4DIGIT
              ; day month year (e.g., 02 Jun 1982)
date2        = 2DIGIT "-" month "-" 2DIGIT
              ; day-month-year (e.g., 02-Jun-82)
date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
              ; month day (e.g., Jun  2)
time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
              ; 00:00:00 - 23:59:59
wkday        = "Mon" | "Tue" | "Wed"
            | "Thu" | "Fri" | "Sat" | "Sun"
weekday      = "Monday" | "Tuesday" | "Wednesday"
            | "Thursday" | "Friday" | "Saturday" | "Sunday"
month        = "Jan" | "Feb" | "Mar" | "Apr"
            | "May" | "Jun" | "Jul" | "Aug"
            | "Sep" | "Oct" | "Nov" | "Dec"

File a bug with the service you are using. It is invalid to send Last-Modified or If-Modified-Since with milliseconds.

If sub-second accuracy is important, it may be more appropriate to use entity tags (ETag).