Set Expires when the Content of the HTTP response message contains information that may become invalid with the passage of time. Setting an Expires value does not imply that the information or its source will necessarily change or cease to exist when the Expires date and time is reached. Web clients are simply warned to check the freshness of the information after the Expires date and time. 

The value of Expires is specified in Greenwich Mean Time. 

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

Date 

LastModified 

Describing the Content