Based on the interface of the DMC, it appears that we can go back only 24 hours when rebuilding the forwarder assets. I just did it in our production environment and only one forwarder is reported as missing. I'm pretty sure though that other forwarders are down for more than 24 hours. What can be done?
Are you trying to collect the data from the missing forwarders or are you trying to add the forwarders to the Distributed Management Console.
This process deletes the sourcetype holding all of the existing forwarders and the process is usesually ONLY run oneself or so to clean up an environment where you have many forwarders missing and only want to see existing forwarders. Once you run this process, any forwarders that are no longer reporting are just gone and so is any data about them. This is not retrievable. The 24 hour question is asking how much back data for each forwarder do you want to collect. It cannot collect any data from forwarders that it no longer has any record of.