Docs: Document the HrTime format (#132)

src/common/Time.ts

@@ -13,6 +13,18 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/**
+ * Defines High-Resolution Time.
+ *
+ * The first number, HrTime[0], is UNIX Epoch time in seconds since 00:00:00 UTC on 1 January 1970.
+ * The second number, HrTime[1], represents the partial second elapsed since Unix Epoch time represented by first number in nanoseconds.
+ * For example, 2021-01-01T12:30:10.150Z in UNIX Epoch time in milliseconds is represented as 1609504210150.
+ * The first number is calculated by converting and truncating the Epoch time in milliseconds to seconds:
+ * HrTime[0] = Math.trunc(1609504210150 / 1000) = 1609504210.
+ * The second number is calculated by converting the digits after the decimal point of the subtraction, (1609504210150 / 1000) - HrTime[0], to nanoseconds:
+ * HrTime[1] = Number((1609504210.150 - HrTime[0]).toFixed(9)) * 1e9 = 150000000.
+ * This is represented in HrTime format as [1609504210, 150000000].
+ */
 export type HrTime = [number, number];
 
 /**