Eighteen Months

Robert Anderson realizes that Jack Havig is now an experienced young man, no longer the youth that he had been just the previous year. After a family quarrel, Jack had left home for a month but had spent a year and a half in different future periods. As a result:

"Both his appearance and his demeanor were shockingly changed."
-Poul Anderson, There Will be Time (New York, 1973), II, p. 23.

Maybe Jack delayed his return for a month so that the visible changes in him would be slightly less shocking? Also, Jack's step-father would need a cooling off period after the quarrel.

At last, Jack demonstrates time travel but only to Anderson. First, he disappears for fifteen minutes, i.e., travels fifteen minutes into the future, then he stands beside himself for a minute, i.e., travels one minute into the past.

When Jack initially claimed to have visited the future, Anderson asked him if he had "'...a time machine.'" (p. 36) This terminology is from Wells and Jack will meet Wallis who gave the time travel idea to Wells.

Jack does not know how time travel works any more than he knows how his muscles work. He wills himself pastward or futureward just as he wills to pick up a glass. This makes it sound very natural.

Doctor Anderson knows how muscles work but Jack points out that:

"'...you'll agree your scientific information is only a glimmer on the surface of a mystery.'" (ibid.)

Again, this makes the idea plausible. Time travel is mysterious but then so is everything...