service/prometheus.md

@@ -1423,6 +1423,15 @@ tests:
         #  need to test?). You can play with the eval_time in other test
         #  entries to evaluate the same alert at different offsets in the
         #  timeseries above.
+        #
+        # Note that the `time()` function returns zero when the evaluation
+        # starts, and increments by `interval` until `eval_time` is
+        # reached, which differs from how things work in reality,
+        # where time() is the number of seconds since the
+        # epoch.
+        #
+        # in other words, this means the simulation starts at the
+        # Epoch and stops (here) an hour later.
         - eval_time: 60m
           alertname: NeedsReboot
           exp_alerts: