This Metric is using the time_since function which will allow you to convert to your desired output. You can substitute 'seconds', 'minutes', 'hours', 'days', 'weeks', 'months', or 'years' where 'days' currently sits in the metric.

Endpoints[?lastSeenDays_r > ].join(' ',[hostname, '- Days since checked in:', to_string(time_since(lastSeenAt,'days'))])