It pretty much says it in a title. Interrupt time = 100ns intervals since the OS booted up. And FILETIME = 100ns intervals since midnight of Jan 1, 1601 (UTC.)
You can't, really. In a given driver, you can grab current interrupt time and file time during initialization and save the delta somewhere. That way, you have a conversion factor you can use any time.
I need this for a crash dump. I wonder if they cache that delta somewhere during the boot sequence?