Reproducible hotspot builds were covered by JDK-8241271. However, right now I noticed the following piece of code:

# Always recompile abstract_vm_version.cpp if libjvm needs to be relinked. This ensures
# that the internal vm version is updated as it relies on __DATE__ and __TIME__
# macros.
ABSTRACT_VM_VERSION_OBJ := $(JVM_OUTPUTDIR)/objs/abstract_vm_version$(OBJ_SUFFIX)
$(ABSTRACT_VM_VERSION_OBJ): $(filter-out $(ABSTRACT_VM_VERSION_OBJ) $(JVM_MAPFILE), \
    $(BUILD_LIBJVM_TARGET_DEPS))

This means that even with --with-hotspot-build-time, while we do get reproducible builds, we will also always unnecessarily recompile abstract_vm_version. We should fix that.