Relative tense and absolute tense are distinct possible uses of the grammatical category of tense. Absolute tense means the grammatical expression of time reference (usually past, present or future) relative to "now" – the moment of speaking. In the case of relative tense, the time reference is construed relative to a different point in time, the moment being considered in the context. In other words, the reference point (or center of deixis) is the moment of discourse or narration in the case of absolute tense, or a different moment in the case of relative tense.
A further distinction has also been made between "strict relative" tense, which merely expresses time relative to the reference point, and "absolute-relative tense" (such as pluperfect), which expresses time relative to the reference point while also placing the reference point in time relative to the present moment.
A relative past tense is sometimes called an anterior tense, while a relative future tense may be called a posterior tense.
I have looked up the definitions of the past tense and present perfect tense in John Warriner's magisterial textbook Warriner's English Grammar and Composition. To me, it seems as though Warriner's definition of the present perfect tense is inadequate.
Warriner's definition of the past tense...