Use Expires to determine whether the request is "stale." Expires is the date and time at which the request becomes invalid. The value of Expires is translated into local time as specified by the system clock from the time units set by the Web client. 

If the request has a CacheControl value with a max-age directive, the max-age directive overrides the value of Expires. 

Expires is represented using the TDateTime type. The integral part of a TDateTime value is the number of days that have passed since Dec. 30, 1899. The fractional part of a TDateTime value is the fraction of a 24-hour day that has elapsed. 

Date 

IfModifiedSince 

CacheControl