added system.time to manipulating timestamps guide, resolves #564

content/v2.0/query-data/guides/manipulate-timestamps.md

@@ -59,7 +59,7 @@ to convert the duration to a string.
 
 ## Time-related Flux functions
 
-### Retrieve the current time
+### Retrieve the current UTC time
 Use the [`now()` function](/v2.0/reference/flux/stdlib/built-in/misc/now/) to
 return the current UTC time in RFC3339 format.
 
@@ -67,6 +67,26 @@ return the current UTC time in RFC3339 format.
 now()
 ```
 
+{{% note %}}
+`now()`  is cached at runtime, so all instances of `now()` in a Flux script
+return the same value.
+{{% /note %}}
+
+### Retrieve the current system time
+Import the `system` package and use the [`system.time()` function](/v2.0/reference/flux/stdlib/system/time/)
+to return the current system time of the host machine in RFC3339 format.
+
+```js
+import "system"
+
+system.time()
+```
+
+{{% note %}}
+`system.time()` returns the time it is executed, so each instance of `system.time()`
+in a Flux script returns a unique value.
+{{% /note %}}
+
 ### Add a duration to a timestamp
 The [`experimental.addDuration()` function](/v2.0/reference/flux/stdlib/experimental/addduration/)
 adds a duration to a specified time and returns the resulting time.