Which When?

How does a time traveler control whether he travels futureward or pastward? In a temporal vehicle, like the Time Machine or a Time Patrol timecycle, there are futureward and pastward switches but what of time travelers who do not use vehicles?

The characters in Jack Finney's two Time novels travel only to a period that they can visualize and imagine themselves into, either a past period that they have specialized in or their present as they return to it. The future is as blank to them as it is to us. Audrey Niffenegger's time traveler has no control over either departure or arrival times - but they are important times in his or his wife's life. By the end of the novel, he has died but had told her that he will visit her again before her death.

Poul Anderson's Jack Havig:

"'No, I don't know how it works... But then, I don't know how my muscles work...'"
-Poul Anderson, There Will Be Time (New York, 1973), IV, p. 36.

"'I will myself backward or forward in time...the way I will to, oh, pick my glass off your desk. In other words, I order whatever-it-is to move me, the same as we order our fingers to do something, and it happens.'"
-op. cit., p. 37.

We walk forward or backward. He time travels forward or backward. There is no further explanation.

I have come across a joke (I think) answer. Superman's powers grew impossibly over years and decades. These were discrete stages:

he could run fast and jump high;
he could fly fast;
he could fly faster than light;
faster than light travel could become time travel;
his powers were scaled way back down again (although they still included speed and flight).

How did he change FTL flight into time travel? I don't know. How did he choose between past and future? According to an editorial answer in a loccol (letter of comment column), he rotated clockwise for future and anti-clockwise for past. So now we know.

I wish Anderson had written Superman. He would either (probably) have avoided the time travel or explained it ingeniously.