epoch difference in Makefiles

Below is a part of my Makefile (has errors). I just want the difference between the epoch and the epoch max and print them. But I am not sure what the mistake is. Could someone help me with this. Thanks

EPOCH_MAX = 1400000000

identifier:
    epoch = $(shell date +%s)
    echo $(epoch)
    residue = $(epoch)-$(EPOCH_MAX) 
    echo $(residue)

I get the following error

epoch = 1400767572
make: epoch: Command not found
make: *** [identifier] Error 127

Solution

You are mixing up Makefile variables and shell variables. The following code is what you probably want:

  EPOCH_MAX = 1400000000
  epoch = $(shell date +%s )

  identifier:
        echo $(epoch)
        (( residue = $(epoch) - $(EPOCH_MAX) )) || true ;\
        echo $${residue}

EDIT: As MadScientist stressed, this isn't portable at all. Better is e.g.:

  EPOCH_MAX = 1400000000

  identifier:
      epoch=$(shell date +%s ); \
      echo $$epoch; \
      residue=`expr $$epoch - $(EPOCH_MAX)`; \
      echo $$residue