Arthur Miller's Death of a Salesman is a realistic play that resorts to expressionism in order to portray the lives and feelings of the characters. In other words, the accurate, detailed description of what happens in their lives (realistic elements) is accompanied by sequences in which the characters reminisce about the past or play, in their minds, imaginary situations that show their emotions and concerns (expressionistic elements).
In fact, the two devices you mention -fluctuation in time and foreshadowing- are closely related in this play. However, the former takes place only if you connect what happens in the characters' minds, played out on the stage, with the actual time of the play, that is to say, the present.
An example of this is the following dialogue in Act I:
WILLY: Cause I get so lonely — especially when business is bad
and there’s nobody to talk to. I get the feeling that I’ll never
sell anything again, that I won’t make a living for you, or a
business, a business for the boys. (He talks through The
Woman’s subsiding laughter; The Woman primps at the »mirror«.) There’s so much I want to make for...
THE WOMAN: Me? You didn’t make me, Willy. I picked you.
WILLY (pleased): You picked me?
THE WOMAN: (who is quite proper-looking, Willy’s age): I did.
I’ve been sitting at that desk watching all the salesmen go by,
day in, day out. But you’ve got such a sense of humor, and we
do have such a good time together, don’t we?
WILLY: Sure, sure. (He takes her in his arms.) Why do you have
to go now?
THE WOMAN: It’s two o’clock...
WILLY: No, come on in! (He pulls her.)
THE WOMAN:... my sisters’ll be scandalized. When’ll you be
WILLY: Oh, two weeks about. Will you come up again?
THE WOMAN: Sure thing. You do make me laugh. It’s good for
me. (She squeezes his arm, kisses him.) And I think you’re a
WILLY: You picked me, heh?
THE WOMAN: Sure. Because you’re so sweet. And such a kidder.
This is something Willy remembers. He has been unfaithful to Linda, and feels guilty about it. So the line
I’ll make it all up to you, Linda, I’ll...
is associated to his present feeling of guilt about a past event that has come to his memory triggered by Linda's laugh, which connected, in his mind, to this other woman's laugh at the time of his infidelity. Linda is ignorant of this and thinks he is talking about their present predicament.
On the other hand, the bits and pieces that go through Willy's mind in the two days previous to his suicide, added to the way in which his sons address him during some exchanges, foreshadow the ending.
It is clear that Miller intended to convey what can be expected when a man's life is based on illusion only.