diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index f344624666a6c..b8a1fe9910540 100644
--- a/src/librustc_driver/driver.rs
+++ b/src/librustc_driver/driver.rs
@@ -660,6 +660,15 @@ pub fn phase_2_configure_and_expand_inner<'a, F>(sess: &'a Session,
         disambiguator,
     );
 
+    if sess.opts.incremental.is_some() {
+        time(time_passes, "garbage collect incremental cache directory", || {
+            if let Err(e) = rustc_incremental::garbage_collect_session_directories(sess) {
+                warn!("Error while trying to garbage collect incremental \
+                       compilation cache directory: {}", e);
+            }
+        });
+    }
+
     // If necessary, compute the dependency graph (in the background).
     let future_dep_graph = if sess.opts.build_dep_graph() {
         Some(rustc_incremental::load_dep_graph(sess, time_passes))
diff --git a/src/librustc_incremental/lib.rs b/src/librustc_incremental/lib.rs
index b53ee1daada42..65fbd9d0bf8f1 100644
--- a/src/librustc_incremental/lib.rs
+++ b/src/librustc_incremental/lib.rs
@@ -46,3 +46,4 @@ pub use persist::in_incr_comp_dir;
 pub use persist::prepare_session_directory;
 pub use persist::finalize_session_directory;
 pub use persist::delete_workproduct_files;
+pub use persist::garbage_collect_session_directories;
diff --git a/src/librustc_incremental/persist/fs.rs b/src/librustc_incremental/persist/fs.rs
index f4171f951f407..795825f180c9d 100644
--- a/src/librustc_incremental/persist/fs.rs
+++ b/src/librustc_incremental/persist/fs.rs
@@ -603,7 +603,7 @@ fn timestamp_to_string(timestamp: SystemTime) -> String {
 }
 
 fn string_to_timestamp(s: &str) -> Result<SystemTime, ()> {
-    let micros_since_unix_epoch = u64::from_str_radix(s, 36);
+    let micros_since_unix_epoch = u64::from_str_radix(s, INT_ENCODE_BASE as u32);
 
     if micros_since_unix_epoch.is_err() {
         return Err(())
@@ -733,6 +733,20 @@ pub fn garbage_collect_session_directories(sess: &Session) -> io::Result<()> {
                                 })
                                 .collect();
 
+    // Delete all session directories that don't have a lock file.
+    for directory_name in session_directories {
+        if !lock_file_to_session_dir.values().any(|dir| *dir == directory_name) {
+            let path = crate_directory.join(directory_name);
+            if let Err(err) = safe_remove_dir_all(&path) {
+                sess.warn(&format!("Failed to garbage collect invalid incremental \
+                                    compilation session directory `{}`: {}",
+                                    path.display(),
+                                    err));
+            }
+        }
+    }
+
+    // Now garbage collect the valid session directories.
     let mut deletion_candidates = vec![];
     let mut definitely_delete = vec![];
 
diff --git a/src/librustc_incremental/persist/mod.rs b/src/librustc_incremental/persist/mod.rs
index 82a43d85bc608..2f864aaefba89 100644
--- a/src/librustc_incremental/persist/mod.rs
+++ b/src/librustc_incremental/persist/mod.rs
@@ -20,9 +20,10 @@ mod save;
 mod work_product;
 mod file_format;
 
-pub use self::fs::prepare_session_directory;
 pub use self::fs::finalize_session_directory;
+pub use self::fs::garbage_collect_session_directories;
 pub use self::fs::in_incr_comp_dir;
+pub use self::fs::prepare_session_directory;
 pub use self::load::dep_graph_tcx_init;
 pub use self::load::load_dep_graph;
 pub use self::load::load_query_result_cache;