Docs: Document the HrTime format (#2604)

Author: JamesJHPark

packages/opentelemetry-core/src/common/time.ts

@@ -22,7 +22,15 @@ const NANOSECOND_DIGITS = 9;
 const SECOND_TO_NANOSECONDS = Math.pow(10, NANOSECOND_DIGITS);
 
 /**
- * Converts a number to HrTime
+ * Converts a number to HrTime, HrTime = [number, number].
+ * The first number is UNIX Epoch time in seconds since 00:00:00 UTC on 1 January 1970.
+ * The second number 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.
+ * numberToHrtime calculates the first number by converting and truncating the Epoch time in milliseconds to seconds:
+ * HrTime[0] = Math.trunc(1609504210150 / 1000) = 1609504210.
+ * numberToHrtime calculates the second number 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)) * SECOND_TO_NANOSECONDS = 150000000.
+ * This is represented in HrTime format as [1609504210, 150000000].
  * @param epochMillis
  */
 function numberToHrtime(epochMillis: number): api.HrTime {