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Note: When using the If-Modified-Since header it is recommended that you use a date retrieved from the Last-Modified header of a previous request to the same service. The reason for this is that the service expects that the parsed date retrieved from the If-Modified-Since-header is an exact match (with millisecond precision) to the date used in the Last-Modified header. The service still respect and parses all the date-formats mentioned in http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.3.1.
NOTE ON TESTING THIS: In the test environment we are currently testing the new backend for OPDS generation that will give significant performance improvements. If you at times get an empty feed back, it probably means we are testing the pre generation and have been forced to regenerate the feed. This is just a scenario in the testing process, if this happens and remains a problem please contact us.