Convert a 13-digit string Unix timestamp tag to integer

I have an InfluxDB tag, named completionTime, which is a 13-digit string Unix timestamp in a Grafana query.
My Grafana version does not support Flux, unfortunately.
I would like to compare the completionTime tag values to ${__from} and ${__to}, which are integers (as far as I know). So, I need to convert the completionTime tag values to integer values to be able to add the comparison to my Grafana query.

Please help.