[In the future, please direct questions or error reports where you are not sure whether it is "Brian's fault" to Brian's support mailing list (briansupport.com), we try to keep the github issues for bugs and feature requests.]
You can use
t to refer to the current time, and
lastspike for the time of the last spike, so
threshold='1/(t-lastspike)' should do what you want I think.