You no longer need to do a timestamp conversion, you can use timedelta instead:
alias: alexa alarm
description: test
trigger:
- platform: template
value_template: >
{% set alarm_time = states('sensor.echo_dot_next_alarm') | as_datetime %}
{{ now() >= alarm_time - timedelta(seconds = 2) }}
action:
- service: notify.alexa_media_echo_dot
data:
message: alarm will trigger soon
data:
type: tts
mode: single