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I have given this support before, the relevant date is the visa issue date and not the entry date.
There is somewhere I will have to locate it.
Sorry cant find it but you will see a similar guidance under the Tier 2 policy document for ILR at paragraph 263.
YES the policy for main applicants also applies to dependants.
The information you received form the Home Office is correct " ... first time i heard that it will be from visa start date. calculating from application date back wards till five years and any absence from visa start date and entry date will be calculated to 180 days absence."
Please note that you can only apply NO MORE THAN 28 days before.
We must advice those using JustAnswer that they must seek legal advice before making expensive applications and that JustAnswer does not give legal advice.