This Doesn't Work

INPUTS := $(wildcard *.in)
OUTPUTS := $(INPUTS:%.in=%.out)
DATE := $(shell date +%Y-%m-%d)

all: $(OUTPUTS)

%.out: %.in %.$(DATE).timestamp
    @cat $< > $@

%.$(DATE).timestamp:
    @echo $(DATE) > $@

clean:
    @rm -f *.out
    @rm -f *.timestamp

.PHONY: clean