Don't redownload “latest” casks unnecessarily.

Library/Homebrew/cask/cask.rb

@@ -80,6 +80,13 @@ def installed?
       !versions.empty?
     end
 
+    def install_time
+      _, time = timestamped_versions.last
+      return unless time
+
+      Time.strptime(time, Metadata::TIMESTAMP_FORMAT)
+    end
+
     def installed_caskfile
       installed_version = timestamped_versions.last
       metadata_master_container_path.join(*installed_version, "Casks", "#{token}.rb")

Library/Homebrew/cask/download.rb

@@ -35,7 +35,7 @@ def downloader
     attr_accessor :downloaded_path
 
     def clear_cache
-      downloader.clear_cache if force || cask.version.latest?
+      downloader.clear_cache if force
     end
 
     def fetch

Library/Homebrew/cask/metadata.rb

@@ -3,6 +3,7 @@
 module Cask
   module Metadata
     METADATA_SUBDIR = ".metadata"
+    TIMESTAMP_FORMAT = "%Y%m%d%H%M%S.%L"
 
     def metadata_master_container_path
       @metadata_master_container_path ||= caskroom_path.join(METADATA_SUBDIR)
@@ -58,12 +59,7 @@ def metadata_subdir(leaf, version: self.version, timestamp: :latest, create: fal
     private
 
     def new_timestamp(time = Time.now)
-      time = time.utc
-
-      timestamp = time.strftime("%Y%m%d%H%M%S")
-      fraction = format("%.3f", time.to_f - time.to_i)[1..-1]
-
-      timestamp.concat(fraction)
+      time.utc.strftime(TIMESTAMP_FORMAT)
     end
   end
 end

Library/Homebrew/download_strategy.rb

@@ -296,6 +296,8 @@ def fetch
 
       fresh = if cached_location.exist? && url_time
         url_time <= cached_location.mtime
+      elsif version.respond_to?(:latest?)
+        !version.latest?
       else
         true
       end