diff --git a/rslib/Makefile b/rslib/Makefile
index 28c759583..42f822b41 100644
--- a/rslib/Makefile
+++ b/rslib/Makefile
@@ -31,8 +31,10 @@ RUST_TOOLCHAIN := $(shell cat rust-toolchain)
 	rustup component add clippy-preview --toolchain $(RUST_TOOLCHAIN)
 	@touch $@
 
+# set TZ here for the benefit of a single unit test that fails
+# when running in the GitHub workflow.
 .build/check: .build/rs-tools $(ALL_SOURCE)
-	cargo test --lib -- --nocapture
+	TZ="Australia/Perth" cargo test --lib -- --nocapture
 	cargo fmt -- --check
 	cargo clippy -- -D warnings
 	@touch $@