With the release of Curl 7.0 there is now an additional resync facility as noted by Chris Barber.
Here is what the documentation says:
resync-file (accessor) Class: ComponentMetaData Package: CURL.LANGUAGE.COMPONENT
getter public sealed resync-file:#String
setter public sealed resync-file:#String
Set component resynchronization time from specified file.
When specified in the meta-data for an applet or script this attribute defines the URL of a file whose when-last-modified time will be used to set the value of process-resync-as-of.
When setting this value through this setter, the value must contain an absolute URL or an exception will be thrown. When setting via the applet or script declaration a relative path may be used or the empty string may be used as shorthand for the main applet or script load file.
