diff --git a/docs/en/setup/README.md b/docs/en/setup/README.md
index 3d21c0cd0..d07b753e4 100644
--- a/docs/en/setup/README.md
+++ b/docs/en/setup/README.md
@@ -2,7 +2,7 @@
 Setup based on which kind of probes are you going to use. If you don't understand, please read [Concepts and Designs](../concepts-and-designs/README.md) first.
 
 
-**Important: Please comfirm the system time on the monitored servers are same as the time on the OAP servers.**
+**Important: Please choose the timezone on UI, make it matches the timezone of your OAP backend servers.**
 
 
 ## Download official releases
diff --git a/docs/en/setup/backend/backend-setup.md b/docs/en/setup/backend/backend-setup.md
index fe42829be..341950e0a 100644
--- a/docs/en/setup/backend/backend-setup.md
+++ b/docs/en/setup/backend/backend-setup.md
@@ -79,7 +79,22 @@ rules targeting the analysis oal metric objects.
 scale and support high payload, you may need this. 
 1. [Metric exporter](metric-exporter.md). Use metric data exporter to forward metric data to 3rd party
 system.
+1. [Time To Live (TTL)](ttl.md). Metric and trace are time series data, they would be saved forever, you could 
+set the expired time for each dimension.
 
 ## Telemetry for backend
 OAP backend cluster itself underlying is a distributed streaming process system. For helping the Ops team,
 we provide the telemetry for OAP backend itself. Follow [document](backend-telemetry.md) to use it.
+
+
+## FAQs
+#### When and why do we need to set Timezone?
+SkyWalking provides downsampling time series metric features. 
+Query and storage at each time dimension(minute, hour, day, month metric indexes)
+related to timezone when doing time format. 
+
+For example, metric time will be formatted like YYYYMMDDHHmm in minute dimension metric,
+which format process is timezone related.
+  
+In default, SkyWalking OAP backend choose the OS default timezone.
+If you want to override it, please follow Java and OS documents to do so.
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diff --git a/docs/en/setup/backend/ttl.md b/docs/en/setup/backend/ttl.md
new file mode 100644
index 000000000..f82b64119
--- /dev/null
+++ b/docs/en/setup/backend/ttl.md
@@ -0,0 +1,19 @@
+# TTL
+In SkyWalking, there are two types of observability data, besides metadata.
+1. Record, including trace and alarm. Maybe log in the future.
+1. Metric, including such as p99/p95/p90/p75/p50, heatmap, success rate, cpm(rpm) etc.
+Metric is separated in minute/hour/day/month dimensions in storage, different indexes or tables.
+
+You have following settings for different types.
+```yaml
+    # Set a timeout on metric data. After the timeout has expired, the metric data will automatically be deleted.
+    recordDataTTL: ${SW_CORE_RECORD_DATA_TTL:90} # Unit is minute
+    minuteMetricsDataTTL: ${SW_CORE_MINUTE_METRIC_DATA_TTL:90} # Unit is minute
+    hourMetricsDataTTL: ${SW_CORE_HOUR_METRIC_DATA_TTL:36} # Unit is hour
+    dayMetricsDataTTL: ${SW_CORE_DAY_METRIC_DATA_TTL:45} # Unit is day
+    monthMetricsDataTTL: ${SW_CORE_MONTH_METRIC_DATA_TTL:18} # Unit is month
+```
+
+- `recordDataTTL` affects **Record** data.
+- `minuteMetricsDataTTL`, `hourMetricsDataTTL`, `dayMetricsDataTTL` and `monthMetricsDataTTL` affects
+metric data in minute/hour/day/month dimensions.
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