Visuddhimagga, XIII, 111-114:
111. "...'Present' (paccapanna) is of three kinds, that is to say, present by moment, present by continuity, and present by extent. Herein, what has reached arising (uppaada), presence (.thitti), and dissolution (bhanga) is present by moment. What is included in one or two rounds of continuity is present by continuity.
112. "Herein, when someone goes to a well-lit place after sitting in the dark, an object is not clear at first; until it becomes clear, one or two rounds of continuity should be understood [to pass] meanwhile. And when he goes into an inner closet after going about in a well-lit place, a visible object is not immediately evident at first; until it becomes clear, one or two rounds of continuity should be understood [to pass] meanwhile. When he stands at a distance, although he sees the alterations (movements) of the hands of the washermen and the alterations (movements) of the striking of the gongs, drums, etc., yet he does not hear the sound at first...; until he hears it, one or two rounds of continuity should be understood [to pass] meanwhile. This, firstly, is according to the Majjhima reciters.
113. "The Sa"myutta reciters, however, say that there are two kinds of continuity, that is to say, material continuity and immaterial continuity: that a material continuity lasts as long as the [muddy] line of water touching the bank when one treads in the water takes to clear, as long as the heat of the body in one who has walked a certain extent takes to die down, as long as the blindness in one who has come from the sunshine into a does not depart, as long as when, after someone has been giving attention to his meditation subject in a room and then opens the shutters by day and looks out, the dazzling in his eyes does not die down; and that immaterial continuity consists in two or three rounds of impulsion. Both of these are [according to them] called 'present by continuity'.
114. "What is delimited by a single becoming (existence) is called present by extent, with reference to which it is said in the Bhaddekaratta Sutta: 'Friends, the mind and mental objects are both what is present. Consciousness is bound by desire and greed for what is present. Because consciousness is bound by desire and greed he delights in that. When he delights in that, then he is vanquished with respect to present states' (M.iii, 197).
And here 'present by continuity' is used in the Commentaries while 'present by extent' is used in the Suttas."
Visuddhimagga XIV, 187-191:
187. Herein, ...firstly, according to extent: in the case of single becoming of one [living being], previous rebirth linking is past, subsequent to death is future, between these two is present.
188. ...According to continuity: that [materiality] which has like or single origination by temperature and single origination by nutriment, though it occurs successively, is present. That which, previous to that, was of unlike origination by temperature and nutriment is past. That which is subsequent is future. That which is born of consciousness and has its origination in one cognitive series, in one impulsion, in one attainment, is present. Previous to that is past. Subsequent to that is future. There is no special classification into past continuity, etc., of that which has its origination in kamma, but its pastness, etc., should be understood according as it supports those which have their origination through temperature, nutriment, and consciousness.
189. ...According to period: any period among those such as one minute, morning, evening, day-and-night, etc., that occurs as a continuity, is called present. Previous to that is past. Subsequent is future.
190. ...According to moment: what is included in the trio of moments, [that is to say, arising, presence, and dissolution] beginning with arising is called present. At a time previous to that it is called future. At a time subsequent to that it is called past.
191: Furthermore, that whose functions of cause and condition have elapsed is past. That whose function of cause is finished and whose function of condition is unfinished is present. That which has not attained to either function is future. Or alternatively, the moment of the function is present. At a time previous to that it is future. At a time subsequent to that it is past.
And here only the explanations beginning with the moment are absolutely literal. The rest are in a figurative [or relative] sense."