Just here:I don't know where you got 10.13.3. Apple offers only 10.13.6 for download and the re-issued version released last October don't need the change of date at all.
Btw, 10.13.3 BootROM version is MP51.0084.B00.
I downloaded it several times and had always the error about content corrupted, which was finally the date issue.
indeed, I first tried with version I installed last year and got same error and thought wrongly my archive was corrupted. Thus went to download it again, got same error and I ended here where I got the hint about the date.
About the ROM, I followed your instruction and used last year a link pointing to 10.13.6, thus my ROM updated to 0089 (so no alert to update the firmware).
About 10.13.3, once I used originally the 10.13.6 to update my ROM (in prevision to move to Mojave), I never downloaded it.
As I did many many trials, and as I was convinced my local record was bad, I needed to delete it to get ‘Download’ from Apple’s page. So I cannot figure out how I could get 10.13.3 if not from my last downloa
My target was to install HS in HFS+ what I succeeded. Getting the 10.13.3 was unexpected but not an issue to solve, only time matter.
Let me take opportunity to thank you about your detailed procedure allowing me to extend the life from my cMP.