Fathwa - Fasting a day to complete(the fasting of) Ramadhaan if people differ concerning fasting it

Question
What is the ruling on fasting a day to completeRamadhaan if people
differ concerning fasting it? When should fasting be observed if this
is valid?
Answer
All perfect praise be to Allaah, The Lord of the worlds. I testify
that there is none worthy of worship except Allaah, and that Muhammad,
is His slave and messenger.
Whoever misses a day of Ramadhaan should make up for it. Making it up
should not be on a specific day but he should make up for it before
the following Ramadhaan. The questioner said: "If people differ
concerning fasting it". This means that people are differing with
regards to the beginning or end of the month. If the country does not
depend, in determining the beginning or end of the month, upon
astronomical calculationsbut rather on an actual sighting and they saw
the moon and fasted twenty nine days, then they have done what they
were ordered to do. Their fasting is correct and the doubt does not
matter.
However, if they depend upon astronomical calculations and their
fasting was less than what it should be according to the moon sighting
or completing Sha'baan, then they should fast the missed days whether
one or twodays.
Allaah Knows best.