# Introduction
`phosphor-time-manager` is the time manager service that implements D-Bus
interface `xyz/openbmc_project/Time/EpochTime.interface.yaml`.
The user can get or set the BMC's time via this interface.

### General usage
The service `xyz.openbmc_project.Time.Manager` provides an object on D-Bus:
* /xyz/openbmc_project/time/bmc

where each object implements interface `xyz.openbmc_project.Time.EpochTime`.

The user can directly get or set the property `Elapsed` 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 BMC's time:
   ```
   ### With busctl on BMC
   busctl set-property xyz.openbmc_project.Time.Manager \
       /xyz/openbmc_project/time/bmc 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/bmc/attr/Elapsed
   ```

### Time settings
Getting BMC time is always allowed, but setting the time may not be
allowed depending on the below two settings in the settings manager.

* TimeSyncMethod
   * NTP: The time is set via NTP server.
   * MANUAL: The time is set manually.

A summary of which cases the time can be set on BMC or HOST:

Mode      | Set BMC Time
--------- | -------------
NTP       | Fail to set
MANUAL    | OK

* To set an NTP [server](https://tf.nist.gov/tf-cgi/servers.cgi):
   ```
   ### With busctl on BMC
   busctl set-property  xyz.openbmc_project.Network \
      /xyz/openbmc_project/network/eth0 \
      xyz.openbmc_project.Network.EthernetInterface NTPServers \
      as 1 "<ntp_server>"

   ### With REST API on remote host
   curl -c cjar -b cjar -k -H "Content-Type: application/json" -X  PUT  -d \
       '{"data": ["<ntp_server>"] }' \
       https://${BMC_IP}/xyz/openbmc_project/network/eth0/attr/NTPServers
   ```

* To go into NTP mode
   ```
   ### With busctl on BMC
   busctl set-property xyz.openbmc_project.Settings \
       /xyz/openbmc_project/time/sync_method xyz.openbmc_project.Time.Synchronization \
       TimeSyncMethod s "xyz.openbmc_project.Time.Synchronization.Method.NTP"

   ### With REST API on remote host
   curl -c cjar -b cjar -k -H "Content-Type: application/json" -X  PUT  -d \
       '{"data": "xyz.openbmc_project.Time.Synchronization.Method.NTP" }' \
       https://${BMC_IP}/xyz/openbmc_project/time/sync_method/attr/TimeSyncMethod
   ```

### Special note on changing NTP setting
Starting from OpenBMC 2.6 (with systemd v239), systemd's timedated introduces
a new beahvior that it checks the NTP services' status during setting time,
instead of checking the NTP setting:

* When NTP server is set to disabled, and the NTP service is stopping but not
   stopped, setting time will get an error.

In OpenBMC 2.4 (with systemd v236), the above will always succeed.

This results in [openbmc/openbmc#3459][1], and the related test cases are
updated to cooperate with this behavior change.

### Special note on host on
When the host is on, the changes of the above time mode are not applied but
deferred. The changes of the mode are saved to persistent storage.

When the host is off, the saved mode are read from persistent storage and are
applied.

Note: A user can set the time mode in the settings daemon at any time,
but the time manager applying them is governed by the above condition.


[1]: https://github.com/openbmc/openbmc/issues/3459