Do calculations with 2 differently formatted date time values

Hi. Under /dev-state i have one entity stating its value as “[‘2018-03-25’, ‘16:29:01’]” (without “”) and datetime reporting its value in the format “2018-03-25, 19:06” (again, without""). I want to know the difference between the 2 entities in hours.

Based on some Google-ing, i now have

    value_template: '{{ (as_timestamp(sensor.date__time) - states.sensor.hall_motion_sensor.attributes.last_updated.split(" ")[0] | int *60 ) | timestamp_custom("%Y-%m-%d %H:%M") }}'

But this does not give a result. The sensor i put this value in reports value “Unknown”.

I don’t know which commands and string handling options i have.