# rclcpp: Time

The _rclcpp::Time_ and _rclcpp::Duration_ are a significant departure from
their ROS 1 equivalents, but are more closely related to
[std::chrono](https://en.cppreference.com/w/cpp/chrono). For an in-depth
discussion comparing with std::chrono, see this discussion on
[ROS Discourse](https://discourse.ros.org/t/ros-2-time-vs-std-chrono/6293).

When porting certain ROS 1 libraries, there may be significant usage of
timestamps as floating-point seconds. To get floating point seconds from
an _rclcpp::Time_:

```cpp
// node is instance of rclcpp::Node
rclcpp::Time t = node.now();
double seconds = t.seconds();
```

There is no constructor for Time from seconds represented by a floating point,
so you first need to convert to nanoseconds:

```cpp
rclcpp::Time t(static_cast<uin64_t>(seconds * 1e9));
```

_rclcpp::Duration_ does have functions to go both directions:

```cpp
rclcpp::Duration d = rclcpp::Duration::from_seconds(1.0);
double seconds = d.seconds();
```

Unlike ROS 1, sleeping does NOT occur from a Duration instance:

```cpp
rclcpp::sleep_for(std::chrono::milliseconds(50));
```

## API Documentation Links

 * [rclcpp::Time](https://docs.ros2.org/latest/api/rclcpp/classrclcpp_1_1Time.html)
 * [rclcpp::Duration](https://docs.ros2.org/latest/api/rclcpp/classrclcpp_1_1Duration.html)