changing timestamp-utility functions to pub (#614)

* changing timestamp-utility functions to pub

As suggested in issue #613, I've moved the timestamp utilities
public. Also physically moved the functions outside the comment
for utility functions for clarity. Will allow the same functionality
to be used for other timestamp data in the task, such as `scheduled`.

* Add documentation comments

Author: martiege

src/task/task.rs

@@ -546,16 +546,11 @@ impl Task {
         self.set_value(key, None, ops)
     }
 
-    // -- utility functions
-
-    fn is_known_key(key: &str) -> bool {
-        Prop::from_str(key).is_ok()
-            || key.starts_with("tag_")
-            || key.starts_with("annotation_")
-            || key.starts_with("dep_")
-    }
-
-    fn get_timestamp(&self, property: &str) -> Option<Timestamp> {
+    /// Get the given timestamp property.
+    ///
+    /// This will return `None` if the property is not set, or if it is not a valid
+    /// timestamp. Otherwise, a correctly parsed Timestamp is returned.
+    pub fn get_timestamp(&self, property: &str) -> Option<Timestamp> {
         if let Some(ts) = self.data.get(property) {
             if let Ok(ts) = ts.parse() {
                 return Some(utc_timestamp(ts));
@@ -565,14 +560,24 @@ impl Task {
         None
     }
 
-    fn set_timestamp(
+    /// Set the given timestamp property, mapping the value correctly.
+    pub fn set_timestamp(
         &mut self,
         property: &str,
         value: Option<Timestamp>,
         ops: &mut Operations,
     ) -> Result<()> {
         self.set_value(property, value.map(|v| v.timestamp().to_string()), ops)
     }
+
+    // -- utility functions
+
+    fn is_known_key(key: &str) -> bool {
+        Prop::from_str(key).is_ok()
+            || key.starts_with("tag_")
+            || key.starts_with("annotation_")
+            || key.starts_with("dep_")
+    }
 }
 
 #[cfg(test)]