Are the simple past and present perfect tenses identical in their time relationship to the present?
The simple past tense refers to an action that has been completed and has no connection to the present; the present perfect tense refers to an action that has just been completed in proximity to the present.
Hence the simple past will imply a time in the past during which the action was completed; for example:
I swam across the English Channel twice.
The present perfect, on the other hand, will imply an action that has just been completed as the sentence is made; for example:
I have swum the English Channel.