As @alemabrahao mentioned you can use the License Calculator to figure out exactly how adding a license will impact the co-term date. Here's an example of the tool modeling current expiration of Sept 22, 2027 and the impact of adding a 3 year renewal license. It would move the co-term date to Sept 21, 2030. When using the tool always start with your current org license state on line 1 (license level, mode "add", default start & claim date of current date, then add the device models and quantities. Sept 22, 2027 is 755 days from today (Aug 28, 2025). So I used 755 days as the term on line 1. Line 2 is where you can simulate a new license or type Add or Renewal. In your example of just wanting to extend time you would want the type to be renewal. If you wanted to both add devices and time you'd use a mode type of Add.
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