{% if now().hour >= 12 %}
Not exactly the same, but close enough. Or you could do:
{% set n = now() %}
{% if n.hour > 12 or n.hour == 12 and (n.minute or n.second or n.microsecond) %}
Also, the usual warnings about automatic entity extraction.
EDIT: Actually to be closest to what you had:
{% set n = now() %}
{% if n.hour > 12 or n.hour == 12 and n.minute %}