parse expiry date with coreutils' date command to allow specification of duration / interval

Makefile

@@ -4,7 +4,7 @@ gpgsm-params : gpgsm-params-template subject-keygrip.hex issuer-keygrip.hex expi
 	./prepare-gpgsm-params --template gpgsm-params-template \
 		--subject-keygrip "$(shell cat subject-keygrip.hex)" \
 		--issuer-keygrip "$(shell cat issuer-keygrip.hex)" \
-		--expiry-datetime "$(shell cat expiry-datetime)" \
+		--expiry-datetime "$(shell date --date="$(shell cat expiry-datetime)")" \
 		> $@
 
 cert.der : gpgsm-params

expiry-datetime

@@ -1 +1 @@
-2017-07-02T12:03:28+02:00
++1 month