diff --git a/lib/upstream.js b/lib/upstream.js
index eeed0a0..237908c 100644
--- a/lib/upstream.js
+++ b/lib/upstream.js
@@ -39,8 +39,11 @@ async function getUpstreamRegistry(repoUrl) {
     // checkout each historical version:
     const versionsDups = await [ headLog, ...oldSchemaLogs ].reduce(async (aProm, log, i) => {
       const a = await aProm
-      const [ hash, commitUnixTime ] = log.split(' ') // see "%h %at"above
+      const [ hash, _ ] = log.split(' ') // see "%h %at"above
       await execFile('git', [ "checkout", hash ], { cwd: REPO_PATH, error: die })
+      const commitUnixTime = (await execFile('git', [
+        'log', '-1', '--pretty=format:%at', '--', path.join(DATA_PATH, `${name}.json`)
+        ], { cwd: REPO_PATH, error: die })).stdout
       const data = JSON.parse(await readFile(path.join(DATA_PATH, `${name}.json`)))
       const { $id } = JSON.parse(await readFile(path.join(SCHEMA_PATH, schemaFile)))
       const schemaVersion = path.basename($id)