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You are asking about this page of the application form...
This is strictly indicative information that they use to help understand the other parts of your application. It has no other value once a visa has been issued.
The actual start and end dates of your visa are printed on the visa itself and travel between those dates (inclusively) is fine.
NOTE: if the form you are filling out does NOT look like the one above, then you are filling out the wrong form. For the remainder of 2017, the starting page for a Standard Visitor Visa is at https://visas-immigration.service.gov.uk/product/uk-visit-visa