Why is repeat.index starting at 12?

Configuration
,
1 
alias: Calendar Export Monthly
description: ""
triggers:
  - trigger: calendar
    entity_id: calendar.system_automation
    event: start
    offset: "0:0:0"
conditions:
  - condition: state
    entity_id: calendar.system_automation
    attribute: message
    state: "Export: Calendar"
actions:
  - sequence:
      - variables:
          endts: "{{ as_timestamp(states('sensor.date_end_last_month'))|float }}"
          enddate: "{{ as_datetime(endts + 86400).strftime('%Y-%m-%d') }}"
          startts: "{{ as_timestamp(as_datetime(endts).strftime('%Y-%m-01'))|float }}"
          startdate: "{{ as_datetime(startts).strftime('%Y-%m-%d') }}"
          days: "{{ ((endts - startts)/86400)+1|int }}"
      - repeat:
          sequence:
            - variables:
                index: "{{ repeat.index|int -1}}"
                day_offset: "{{ index * 86400 }}"
                dt: >-
                  {{ as_datetime(startts + day_offset).strftime('%A %d-%B-%Y')
                  }}
          count: "{{ days|int }}"
mode: single
2

FYI, this didn’t get hits because you put it into development, which is a section about writing code for HA core. It’s not a section about writing automations. I’m going to update the tags to the proper section to get more hits from other users.

3

On to your problem, please verify that the index is actually starting at 12. You can do this by modifying the code this way:

alias: Calendar Export Monthly
description: ""
triggers:
  - trigger: calendar
    entity_id: calendar.system_automation
    event: start
    offset: "0:0:0"
conditions:
  - condition: state
    entity_id: calendar.system_automation
    attribute: message
    state: "Export: Calendar"
actions:
  - sequence:
      - variables:
          endts: "{{ as_timestamp(states('sensor.date_end_last_month'))|float }}"
          enddate: "{{ as_datetime(endts + 86400).strftime('%Y-%m-%d') }}"
          startts: "{{ as_timestamp(as_datetime(endts).strftime('%Y-%m-01'))|float }}"
          startdate: "{{ as_datetime(startts).strftime('%Y-%m-%d') }}"
          days: "{{ ((endts - startts)/86400)+1|int }}"
      - repeat:
          sequence:
            - variables:
                index: "{{ repeat.index|int -1}}"
                day_offset: "{{ index * 86400 }}"
                dt: >-
                  {{ as_datetime(startts + day_offset).strftime('%A %d-%B-%Y')
                  }}
          - action: persistent_notification.create
            data:
              message: Repeat Index {{ repeat.index }}
          count: "{{ days|int }}"
mode: single

Now your automation can also be optimized, there’s a couple ways to remove some of the math.

  1. Use as_datetime & as_local over using as_timestamp. It’s easier for math.
  2. Use timedeltas to offset datetime objects
  3. Variables assign types to items they resolve, there’s no reason to cast things with | int or |float if the result is coming from a variable or the repeat object. The type will already be converted. The only caveat to this rule is states() are always strings and you must cast them.
alias: Calendar Export Monthly
description: ""
triggers:
  - trigger: calendar
    entity_id: calendar.system_automation
    event: start
    offset: "0:0:0"
conditions:
  - condition: state
    entity_id: calendar.system_automation
    attribute: message
    state: "Export: Calendar"
actions:
  - sequence:
      - variables:
          end: >
            {{ (states('sensor.date_end_last_month') | as_datetime | as_local).isoformat() }}
          start: >
            {{ (end | as_datetime).replace(day=1).isoformat() }}
          days: >
            {{ (end | as_datetime).day }}
      - repeat:
          sequence:
            - variables:
                date: >-
                  {{ (start | as_datetime + timedelta(days=repeat.index)).strftime('%A %d-%B-%Y') }}
          count: "{{ days }}"
mode: single

Lastly, if you’re really struggling with repeat.index you can completely skip it by generating your list ahead of time in jinja and using a for_each repeat.

alias: Calendar Export Monthly
description: ""
triggers:
  - trigger: calendar
    entity_id: calendar.system_automation
    event: start
    offset: "0:0:0"
conditions:
  - condition: state
    entity_id: calendar.system_automation
    attribute: message
    state: "Export: Calendar"
actions:
  - sequence:
      - variables:
          dates: >
            {% set end = states('sensor.date_end_last_month') | as_datetime | as_local %}
            {% set start = end.replace(day=1) %}
            [{% for i in range((end - start).days + 1) %}
              "{{ (start + timedelta(days=i)).isoformat() }}",
            {% endfor %}]
      - repeat:
          sequence:
            - variables:
                date: >-
                  {{ (repeat.item | as_datetime).strftime('%A %d-%B-%Y') }}
          for_each: "{{ dates }}"
mode: single