The On Time Tolerance settings allow you to specify the range of early/late in supplier receipts that you're willing to consider as "Close enough" to be considered as on-time.
Access it by going to Admin --> System Configuration --> Default On Time Tolerance Settings
Typically, this is set to a max of 5 days early and 2 days late, on the logic that being a little bit late can be very disruptive to planning, but being FAR too early can also be an issue as well.
Updating these settings affects what is shown as Early / On-Time / Late in the Vendor Scorecard and Lead Time Viewer in the Supply module.