A further enhancements which hasn’t got the drawback of wrong number of days:
value_template: >-
{% set uptime = (value | int) / 100 | int %}
{% set days = (uptime / 86400) | int %}
{%- if days > 0 -%}
{{ days }} days, {{ (uptime - (days * 86400)) | int | timestamp_custom('%H:%M:%S') }}
{%- else -%}
{{ uptime | int | timestamp_custom('%H:%M:%S') }}
{%- endif -%}