The unit of measure is microseconds, not milliseconds.
Backpatch to 9.3 where the function and its comment were added.
/*
* GetCurrentIntegerTimestamp -- get the current operating system time as int64
*
- * Result is the number of milliseconds since the Postgres epoch. If compiled
+ * Result is the number of microseconds since the Postgres epoch. If compiled
* with --enable-integer-datetimes, this is identical to GetCurrentTimestamp(),
* and is implemented as a macro.
*/