Add support for unavailable durations in Duration type

Introduce `DurationUnavailable` constant to represent an indefinite duration. Implement `Duration()` method to return this constant when duration data is unavailable, enhancing the robustness of duration handling.

duration.go

@@ -10,6 +10,10 @@ import (
 	"time"
 )
 
+// DurationUnavailable represents an indefinite duration that is used to indicate that a specific
+// duration is unavailable.
+const DurationUnavailable = time.Duration(-1)
+
 // Duration is a type wrapper of an WeatherData for holding height values in WeatherData
 // (based on meters a default unit)
 type Duration WeatherData
@@ -45,3 +49,17 @@ func (d Duration) Value() float64 {
 	}
 	return d.floatVal
 }
+
+// Duration returns the Duration value as time.Duration type
+//
+// If the Duration is not available in the WeatherData, Duration will return DurationUnavailable instead.
+func (d Duration) Duration() time.Duration {
+	if d.notAvailable {
+		return DurationUnavailable
+	}
+	duration, err := time.ParseDuration(d.String())
+	if err != nil {
+		return DurationUnavailable
+	}
+	return duration
+}