Praise be to Allaah.
The fasting person has to
make up his fast if he breaks the fast when he is uncertain whether the sun
has set, because Allaah says (interpretation of the meaning):
“then complete your Sawm
(fast) till the nightfall”
and night begins when the
sun sets. If he is certain that it is still day he should not break his fast
until he is certain or thinks it most likely that the sun has set, because
the basic principle is that it is still day, and that does not change unless
there is certainty or strong probability.
The fasting person does not
have to make up the fast if he eats or drinks when he is uncertain whether
dawn has come, because Allaah says (interpretation of the meaning):
“and eat and drink until
the white thread (light) of dawn appears to you distinct from the black
thread (darkness of night)”
Allaah says “until …
appears to you” which indicates that it is permissible to eat and drink
before one is certain that dawn has come, and because he was certain that it
was still night, so it is not haraam for him to eat until he becomes certain
that dawn has come, because the basic principle is that it is still night.
See also the answer to
question no. 38543.
And Allaah knows best.