Thank you, that did the job.
Here is my final code where i count the minutes since the last update, in case somebody sees it in the future:
sensor:
- platform: template
sensors:
time_since_last_update:
friendly_name: "random sensor updated"
value_template: >
{% set t = states.sensor.randomsensor.attributes['last_update'] %}
{{((as_timestamp(now()) - as_timestamp(t[6:10] ~ "-" ~ t[3:5] ~ "-" ~ t[0:2] ~ t[10:16]) )/60)|round(0)}}
unit_of_measurement: "Minutes"
icon_template: mdi:timer