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+# Introduction
+`phosphor-time-manager` is the time manager service that implements dbus
+interface `xyz/openbmc_project/Time/EpochTime.interface.yaml`.
+User can get or set the BMC's or HOST's time via this interface.
+
+### General usage
+The service `xyz.openbmc_project.Time.Manager` provides two objects on dbus:
+* /xyz/openbmc_project/time/bmc
+* /xyz/openbmc_project/time/host
+
+where each object implements interface `xyz.openbmc_project.Time.EpochTime`.
+
+User can directly get or set the property `Elasped` of the objects to get or set
+the time. For example on an authenticated session:
+
+* To get BMC's time:
+   ```
+   ### With busctl on BMC
+   busctl get-property xyz.openbmc_project.Time.Manager \
+       /xyz/openbmc_project/time/bmc xyz.openbmc_project.Time.EpochTime Elapsed
+
+   ### With REST API on remote host
+   curl -b cjar -k https://bmc-ip/xyz/openbmc_project/time/bmc
+   ```
+* To set HOST's time:
+   ```
+   ### With busctl on BMC
+   busctl set-property xyz.openbmc_project.Time.Manager \
+       /xyz/openbmc_project/time/host xyz.openbmc_project.Time.EpochTime \
+       Elapsed t <value-in-microseconds>
+
+   ### With REST API on remote host
+   curl -b cjar -k -H "Content-Type: application/json" -X PUT \
+       -d '{"data": 1487304700000000}' \
+       https://bmc-ip/xyz/openbmc_project/time/host/attr/Elapsed
+   ```
+
+### Time settings
+Getting BMC or HOST time is always allowed, but setting the time may not be
+allowed depending on below two settings in settings manager.
+
+* time_mode
+   * NTP: Time is set via NTP server.
+   * MANUAL: Time is set manually.
+* time_owner
+   * BMC: BMC owns the time and can set the time.
+   * HOST: Host owns the time and can set the time.
+   * SPLIT: BMC and Host owns separate time.
+   * BOTH: Both BMC and Host can set the time.
+
+A summary of which cases the time can be set on BMC or HOST:
+Mode      | Owner | Set BMC Time  | Set Host Time
+--------- | ----- | ------------- | -------------------
+NTP       | BMC   | Not allowed   | Not allowed
+NTP       | HOST  | Not allowed   | Not allowed
+NTP       | SPLIT | Not allowed   | OK
+NTP       | BOTH  | Not allowed   | Not allowed
+MANUAL    | BMC   | OK            | Not allowed
+MANUAL    | HOST  | Not allowed   | OK
+MANUAL    | SPLIT | OK            | OK
+MANUAL    | BOTH  | OK            | OK
+
+### Pgood
+When host is on (pgood == 1), the changes of the above time mode/owner are not
+applied but deferred. The changes of the mode/owner are saved to persistent
+storage.
+
+When host is off (pgood == 0), the saved mode/owner are read from
+persistent storage and are applied.
+
+Note: user can set the time mode and owner in settings daemon at any time,
+but time manager applying them is governed by the above condition.