You may specify period during which earlier mirrored files will be used while indexing instead of real downloading. It is very useful when you do some experiments with mnoGoSearch indexing the same hosts and do not want much traffic from/to Internet. If MirrorHeadersRoot is not specified and headers are not stored to local disk then default Content-Type's given in AddType commands will be used.
Default value of the MirrorPeriod is -1, which means "do not use mirrored files".
For time format see Period command description.