Converting a string

i could need some help with this issue, i have.
I have a sensor that gives a string “14:00:00”.
How can i convert this value, to a timestamp, so i can use it in an automation as time

You need the date too so assuming this is ‘today’

the sensor is created and working, but the problem is, i need a decive_class “timestamp”, because i can’t use a normal device in an time automation (that only wants timestamp devices)
How can i convert this value to a timestamp?

image

I donot understand what is the issue… the code above goes to timestamp.
Add device_class = timestamp ?
I just picked random sensor with me

    - name: teststamp
      state: "{{ as_timestamp(state_attr('automation.testing','last_triggered')) | timestamp_local }}"
      device_class: timestamp

this works fine, the problem must have been in the custom timestamp

thanks!