Timing is everything. Not only is picking the right time to do something crucial but recording that time can be equally important. This is especially true in matters of commerce where the proper or shall we say improper dating of documents can have huge financial implications. 
shetar mukdam, a pre-dated document, is invalid in Jewish law. To allow such a document is an invitation to collect money that one is not rightfully entitled to. This occurs when Reuven borrows money from Shimon say on January 1, 2016. Provided the loan is documented Shimon has a lien on all of Reuven’s property that he owned at that time. On February 1 Reuven sells his “mortgaged” land to Levi. Come March 1 Reuven is unable to pay back his loan. Shimon may now collect his loan from Levi as Shimon’s debt predated Levi’s purchase. However the document recording the sale of land is dated to December 1, possibly because that is when the negotiations began. The misdated document “allows” Levi to keep the land preventing Shimon from collecting on his debt. Besides the inherent unfairness of the above, fear of such would stop commerce in its tracks. Thus all predated documents were deemed unenforceable even if there be no third party involved[1].
Such is quite understandable as it relates to commerce but would appear to have little relevance to a get. Six months from now does it really matter if a get was given on 20th of Tevet or the 21st?Apparently it does. 

“[If a get was] written in the day and signed in the day, [written in the] night and signed in the night , [written in the ] night and signed in the day, it is valid. [However if is written in the] day and signed at night it is invalid” (Gittin 17a) as the document is pre-dated if only for a few hours. Immediately the Gemara asks “why was the requirement of proper dating established by gittin”? A get serves to dissolve a marriage allowing the women to remarry and unlike a loan document one cannot use a get to collect funds. Why should we care about the proper dating of the get? Just give the get and move on! Well not so fast. 
It turns out dating a get is actually quite important though Rav Yochanan and Reish Lakish debate the reason why. According to Rav Yochanan it is because of the “daughter of his sister”. This rather cryptic reason is referring to the case where a man marries his niece, “the daughter of his sister” and she has an affair, something that potentially carries with it a death penalty. In order to prevent that possibility the husband writes an undated get and gives it to his wife. If and when she is brought to court the adulteress produces the get claiming she was divorced at the time. While Jewish courts are very pro-active in almost ensuring a death penalty never be given[2] this is not the means to do so. Hence the rabbis insisted on dating the get so that one not get away with having an affair, keeping open the theoretical possibility of a death penalty[3].
Reish Lakish, who interestingly married Rav Yochanan’s sister, disagreed as he noted that “promiscuity is not common” and we need not make special laws for those rare occasions. While we hope such is true - and was more true in Talmudic times than today (something I am not at all certain about) - I find this argument fascinating. Especially so if one accepts the view of Beit Shammai that “one should only divorce his wife if she engaged in promiscuity” (Gittin 90a) along with the Talmudic recommendation that one marry one’s niece (Yevamot 62b-63a). Clearly this may not have been quite as unusual as one may think. 
Nonetheless Reish Lakish claims the need for dating a get is “because of the fruit”. Jewish thought and law (and those Americans who file their tax returns jointly - an option we do not have in Canada) views the assets of the family as one. The husband was deemed the one responsible for the family’s assets and financial well-being (though the wife has the option of opting out - setting up her own bank account and providing for her own needs). He was also allowed to sell the “fruit” i.e. the produce of the assets the wife brought into the marriage. That right ends with the giving of get and from that time on the ex-wife can recover any assets transferred by the husband. Thus for monetary reasons it becomes imperative to know the exact dating of the get.
Rav Yochanan’s concern is that the wife not be held unaccountable while Reish Lakish is concerned the husband be held accountable. While I do not believe anything should be read into this I find it interesting nonetheless.  
[1] Another example of the importance of dating, one that differs from secular law, is that relating to bankruptcy. While secular law pools all creditors of the same class together dividing any available funds equally, Jewish law stipulates that we pay creditors chronologically. Thus the earliest creditor is to be paid 100% of his debt and whatever funds may be left over go to pay the second creditor and if funds still remain the third creditor would be paid and so on and so forth until the funds are depleted. 
[2] Rabbi Akiva went so far as to state that had he been a member of the Sanhedrin he would have ensured that the death penalty effectively be abolished (Makkot 7a). He was not a member of the Sanhedrin because the Sanhedrin had disbanded forty years before the Temple was destroyed so that they not be forced to deal with so many capital crimes. Considering the Mishna was edited some 170 years after the destruction or some 210 years after capital cases were last tried this entire discussion is rather theoretical or shall we say moral. Dating a get no longer (if it every truly did) has any impact in a court of law but the dating may prevent one from pretending adultery did not occur. Hopefully such may at least on occasion prevent one from engaging in an adulterous relationship.  
[3] The Gemara’s reference is to one’s niece presumably because even were the husband to be inclined to “prosecute” his niece he would refrain from doing so out of respect for his sister. At the same time the same argument could be used in almost all cases. It is the rare husband (I hope) who wants to see his wife put to death for having an affair.