timestamp: Redefine timestamp in Data delivery (#274)

Use more precise terms from the HR-TIME spec to define what
`PressureRecord.time` is relative to:
- Any timestamp value obtained from the platform is converted
  to an unsafe moment from the monotonic clock.
- PressureRecord.[[Time]] converts it into a coarsened time relative
  to its global's origin.

Fixes #257

index.html

@@ -1129,17 +1129,27 @@ <h3>Data delivery</h3>
         </li>
         <li>
           Let |state:PressureState| be an [=adjusted pressure state=] given |data| and |source|.
-        </li>
-        <li>
-          Let |timestamp:DOMHighResTimeStamp| be a timestamp representing the time the |data| was
-          obtained from the |relevantGlobal|'s [=platform collector=].
         <aside class="note">
           The |data| sample and mapping between |data| sample, and [=pressure states=],
           is [=implementation-defined=] and may use many different metrics. For instance,
           for CPU, it might consider processor frequency and utilization, as well
           as thermal conditions.
         </aside>
         </li>
+        <li>
+          Let |timestamp| be a platform-specific timestamp converted in an [=implementation-defined=]
+          way to an [=monotonic clock/unsafe current time=] using the same [=monotonic clock=]
+          that is shared by [=environment settings object/time origins=].
+        <aside class="note">
+          The goal of this step is to ensure that a timestamp that may have been relative to
+          a different time origin is converted to a value that can be used in computations with
+          the same [=monotonic clock=] used by the operations described in [[HR-TIME]].
+        </aside>
+        </li>
+        <li>
+          Let |timeValue| be the [=relative high resolution time=] based on |timestamp| and
+          |relevantGlobal|.
+        </li>
         <li>
           [=list/For each=] |observer:PressureObserver| in |relevantGlobal|'s
           [=registered observer list=] for |source|:
@@ -1160,7 +1170,7 @@ <h3>Data delivery</h3>
               Let |record:PressureRecord| be a new {{PressureRecord}} object with its
               {{PressureRecord/[[Source]]}} set to |source|,
               {{PressureRecord/[[State]]}} set to |state|
-              and {{PressureRecord/[[Time]]}} set to |timestamp|.
+              and {{PressureRecord/[[Time]]}} set to |timeValue|.
             </li>
             <li>
               If |observer|.{{PressureObserver/[[AfterPenaltyRecordMap]]}}[source] [=map/exists=]: