Upgrades to Win10 will occasionally reset the Windows install date.
This date stamp, measured down to the second, is part of the data used to identify each machine uniquely and ensures that it is extremely unlikely that your machine ID matches with a random stranger's.
As above:
Use the machine naming feature to give your systems meaningful names.
Retire the oldest item in the list.
If you are still unsure of how to proceed please file a support ticket with x-aviation so that your records can be reviewed before you freeze.