as object, I would like to know when a computer was built and also last OS installation date
OS installation date is ambiguous: what counts as a new installation? If all the files are new after update, is it still the same installation? The "official" setup script may record the date, but this is distribution dependent and most distros won't do that. However, you can always check the earliest date of the files in
/boot/, most of them are updated very rarely.
uname -a will tell you about the kernel, but a lot of distros update the kernel during their updates, and
/boot/ entries with them - is a new kernel a new installation?
For hardware, it's the same problem - which part of hardware do you want to check? Motherboard is probably the best choice: the age of firmware (BIOS/EFI...).
dmidecode will tell you at least about the firmware - BIOS version and date is the closest to what you are asking about. A better formatted and easier to read is
lshw which also tells you about the firmware date. Also try
hwinfo - it doens't tell dates but could be useful someday. For hard drives,
smartctl will tell you a lot - hard drives have self-monitoring capabilities.