Does seeing the waxing moon (mutawwaq) suggest that it is the second night and therefore retroactively establish the first day of the month on the previous day? Does Ayatullah Sayyid al-Sistani agree with this opinion?
There are only four ways to determine the beginning of the month, these are listed below. The position/size of the moon on the second of the month does not determine the first day of the month.
1739. The 1st day of a month is established in the following four ways:
(i) If a person himself sights the moon. (ii) If a number of persons confirm to have sighted the moon and their words assure or satisfy a person. Similarly, every other thing which assures or satisfies him about the moon having being sighted. (iii) If two just (Adil) persons say that they have sighted the moon at night. The first day of the month will not be established if they differ about the details of the new moon. This difference can be either explicit or even implied. For example, when a group of people goes out in search of a new moon and none but two just persons claim to have seen the new moon. Meanwhile among those in the group who did not see it were other just persons who are equally capable and knowledgeable, then the testimony by the first two just persons will not prove the advent of a new month. (iv) If 30 days pass from the first of Sha’ban , the 1st of Ramadhan will be established, and if 30 days pass from the 1st of Ramadhan the 1st of Shawwal will be established.
1742. If the moon is high up in the sky, or sets late, it is not an indication that the previous night was the first night of the month. Similarly, if there is a halo around it, it is not a proof that the new moon appeared in the previous night.
http://www.sistani.org/english/book/48/2281/