fix: make date offset calculation platform agnostic

Makefile

@@ -139,8 +139,10 @@ endif
 	@echo "Copying packaged.yaml to S3"
 	aws s3 cp $(SAM_BUILD_DIR)/$(APP)/$(AWS_REGION)/packaged.yaml s3://$(S3_BUCKET_PREFIX)-$(AWS_REGION)/apps/$(APP)/$(VERSION)/
 	@echo "Fetching objects with prefix: apps/$(APP)/$(VERSION)/ and filtering by last modified date"
-	@current_date=`date -u +"%Y-%m-%dT%H:%M:%SZ"`; \
-	week_ago=`date -u -v-7d +"%Y-%m-%dT%H:%M:%SZ"`; \
+	# calculate a week ago in platform agnostic manner
+	@current_date_seconds=`date -u +"%s"`; \
+	one_week_ago_seconds=$$((current_date_seconds - 7 * 24 * 3600)); \
+	week_ago=`date -u -r "$$one_week_ago_seconds" +"%Y-%m-%d%H:%M:%SZ"`; \
 	objects=`aws s3api list-objects --bucket $(S3_BUCKET_PREFIX)-$(AWS_REGION) --prefix apps/$(APP)/$(VERSION)/ --query "Contents[?LastModified>='$$week_ago'].[Key]" --output text`; \
 	for object in $$objects; do \
 		echo "Setting ACL for object: $$object"; \