diff --git a/src/libfetchers/github.cc b/src/libfetchers/github.cc
index b00609f73..f79446111 100644
--- a/src/libfetchers/github.cc
+++ b/src/libfetchers/github.cc
@@ -195,30 +195,23 @@ struct GitArchiveInputScheme : InputScheme
             {"rev", rev->gitRev()},
         });
 
-        if (auto res = getCache()->lookup(store, lockedAttrs)) {
-            // FIXME
-            //input.attrs.insert_or_assign("lastModified", getIntAttr(res->first, "lastModified"));
-            return {std::move(res->second), input};
-        }
+        if (auto res = getCache()->lookup(store, lockedAttrs))
+            return {std::move(res->second), std::move(input)};
 
         auto url = getDownloadUrl(input);
 
         auto res = downloadFile(store, url.url, input.getName(), true, url.headers);
 
-        //input.attrs.insert_or_assign("lastModified", uint64_t(lastModified));
-
         getCache()->add(
             store,
             lockedAttrs,
             {
                 {"rev", rev->gitRev()},
-                // FIXME: get lastModified
-                //{"lastModified", uint64_t(lastModified)}
             },
             res.storePath,
             true);
 
-        return {res.storePath, input};
+        return {res.storePath, std::move(input)};
     }
 
     std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & input) const override
@@ -227,6 +220,10 @@ struct GitArchiveInputScheme : InputScheme
 
         auto accessor = makeZipInputAccessor(CanonPath(store->toRealPath(storePath)));
 
+        auto lastModified = accessor->getLastModified();
+        assert(lastModified);
+        input2.attrs.insert_or_assign("lastModified", uint64_t(*lastModified));
+
         accessor->setPathDisplay("«" + input2.to_string() + "»");
 
         return {accessor, input2};
diff --git a/src/libfetchers/input-accessor.hh b/src/libfetchers/input-accessor.hh
index a085b1fa5..c1636b20b 100644
--- a/src/libfetchers/input-accessor.hh
+++ b/src/libfetchers/input-accessor.hh
@@ -85,6 +85,13 @@ struct InputAccessor : public std::enable_shared_from_this<InputAccessor>
     virtual std::string showPath(const CanonPath & path);
 
     SourcePath root();
+
+    /* Return the maximum last-modified time of the files in this
+       tree, if available. */
+    virtual std::optional<time_t> getLastModified()
+    {
+        return std::nullopt;
+    }
 };
 
 typedef std::function<RestrictedPathError(const CanonPath & path)> MakeNotAllowedError;
diff --git a/src/libfetchers/zip-input-accessor.cc b/src/libfetchers/zip-input-accessor.cc
index 8b820bbf3..e391e5b71 100644
--- a/src/libfetchers/zip-input-accessor.cc
+++ b/src/libfetchers/zip-input-accessor.cc
@@ -1,6 +1,7 @@
 #include "input-accessor.hh"
 
 #include <zip.h>
+#include <endian.h>
 
 namespace nix {
 
@@ -28,6 +29,8 @@ struct ZipInputAccessor : InputAccessor
     typedef std::map<const char *, struct zip_stat, cmp_str> Members;
     Members members;
 
+    time_t lastModified = 0;
+
     ZipInputAccessor(const CanonPath & _zipPath)
         : zipPath(_zipPath)
     {
@@ -47,10 +50,34 @@ struct ZipInputAccessor : InputAccessor
         for (zip_uint64_t n = 0; n < nrEntries; ++n) {
             if (zip_stat_index(zipFile, n, 0, &sb))
                 throw Error("couldn't stat archive member #%d in '%s': %s", n, zipPath, zip_strerror(zipFile));
+
+            /* Get the timestamp of this file. */
+            #if 0
+            if (sb.valid & ZIP_STAT_MTIME)
+                lastModified = std::max(lastModified, sb.mtime);
+            #endif
+            auto nExtra = zip_file_extra_fields_count(zipFile, n, ZIP_FL_CENTRAL);
+            for (auto i = 0; i < nExtra; ++i) {
+                zip_uint16_t id, len;
+                auto extra = zip_file_extra_field_get(zipFile, i, 0, &id, &len, ZIP_FL_CENTRAL);
+                if (id == 0x5455 && len >= 5)
+                    lastModified = std::max(lastModified, (time_t) le32toh(*((uint32_t *) (extra + 1))));
+            }
+
             auto slash = strchr(sb.name, '/');
             if (!slash) continue;
             members.emplace(slash, sb);
         }
+
+        #if 0
+        /* Sigh, libzip returns a local time, so convert to Unix
+           time. */
+        if (lastModified) {
+            struct tm tm;
+            localtime_r(&lastModified, &tm);
+            lastModified = timegm(&tm);
+        }
+        #endif
     }
 
     ~ZipInputAccessor()
@@ -164,6 +191,11 @@ struct ZipInputAccessor : InputAccessor
 
         return _readFile(path);
     }
+
+    std::optional<time_t> getLastModified() override
+    {
+        return lastModified;
+    }
 };
 
 ref<InputAccessor> makeZipInputAccessor(const CanonPath & path)