The effective date of an entry clearance normally is the same as when the entry clearance gets issued.
However entry clearance officers are advised to try to match the dates with what was requested and can defer the start date of a visa for up to 3 months. Whether this happens in a specific case is not guaranteed.
An applicant needs to make sure they clearly mention their travel plans and possibly request this deferment in the details section. If you don't request deferment of the start date explicitly then you leave it on the discretion of the ECO to assess whether issuing your entry clearance immediately will possibly cause extension applications later on.
Do request it clearly if you need it, even then its not a given.
4. ECB9.4 Validity of entry clearance
The validity of the entry clearance and the leave to enter given are normally the same. The ECO should check the applicant’s visa application form (VAF) for the proposed date of travel in order to prevent unnecessary applications for an extension of stay, and possible claims for a refund of the extension fee.
There are two significant dates on the entry clearance:
The ‘effective’ date which is the date on which the entry clearance is issued (and therefore the date on which the applicant can travel)
The ‘expiry’ date which is the date when the entry clearance will not be valid for travel to the UK, and nor will it confer leave to enter. The ‘expiry’ date for applicants who qualify for ILE should represent the expiry date of the applicant’s passport and does not indicate an expiry of ‘indefinite leave to enter’
5. ECB9.5 What if applicant is not intending to travel straight away to the UK
The ECO should check the applicant’s visa application form (VAF) for the proposed date of travel in order to prevent unnecessary applications for an extension of stay, and possible claims for a refund of the extension fee.
Where an applicant may not intend or be able to travel to the UK immediately following their application they can apply for a visa at any time before the date of travel to the UK, however, we can only defer the start date of the visa up to a maximum 3 months from the date of application
Guidance ECB09: entry clearance vignettes: types, safeguarding and validity