There is a fascinating discussion in the poskim about someone who had a long time to fulfill a duty or mitzva and was an אונס at the last minute. This is called אונס ביום אחרון. Do we say that since he had plenty of time before the אונס he is not considered אונס or perhaps since when he wanted to do it he was אונס we consider him אונס.
The Sefer Ha-aguda says that if someone made a neder to do something within 30 days and had an אונס he is not considered אונס. He proved this from the mishna at the end of Arachin. The halacha is that a person who sells his home in a city that was walled from the time of Yehoshua Bin Nun has one year in which he has the right to redeem it and after which he can no longer do so. When the buyers saw that it was the last day of the year they would hide so that the seller wouldn't be able to find them and they would be able to keep the house forever [note - nothing is forever. We are all going to a place where we must first give up all our possessions prior to entry....:-)]. Hillel made a decree that the seller could avoid this problem and having to get the FBI involved to find the buyer by simply putting the money in a special chamber and thereby redeem his former home. Asks the Aguda - Why was this decree necessary?? Since he was אנוס in his inability to redeem his home, it should remain his even without a special decree.
AAAHHHH!!!! From here we see that an אונס on the last day is not considered a valid אונס [a deathly blow to procrastinators...]. Thus, if not for the decree, the house would remain the buyer's.
The Acharonim spent thousands and thousands of hours on this Aguda [I spent quite a few myself:-)]. Thee Nesivos asked from a gemara at the beginning of Ksubos that says that if a person gives a get and says that if I don't return within thirty days the get should be valid and then attempts to return on the last day but finds a river in the way [read - אונס] the get would not be valid [i.e. he can claim אונס] if not for the fact that such a scenario is common and he should have considered that he would get stuck [and therefore the get IS valid]. We see from this gemara, that in principal, an אונס ביום אחרון is considered an אונס. KASHYAAAAA!!!!:-)
Rav Chaim HaLevi, with his unique powers of dissection, made a distinction between the two cases. There are two types of conditions relating to time. One is where the time frame is just a "book-end" to perform the act. The act must be performed one within that time period, from beginning to end. The other is where the time is part and parcel of the condition and applies every second during the duration of that time frame. In the former instance an אונס at the last second is not considered an אונס because the person had so much time to do the act and shouldn't have waited until the last minute. In the latter instance, since the time frame was part and parcel of the condition and applied every second, if there is an אונס even during the last second, one may claim אונס.
The case cited by the Aguda is an example of where the time frame is merely a book-end, circumscribing the time in which the house must be redeemed. Thus, even if there is an אונס on the last day, since he had so much time beforehand to redeem the house, the seller is not considered אונס. So too in the case where he made a neder to do something within a certain time period and experienced אונס.
In the case of the get, however, he is essentially saying "If I don't come back every second of the next twelve months then the get should be valid" and we see that there was a time when he desired to return but was prevented from doing so, we rule that the condition was not fulfilled [as he can claim אונס] and the get is invalid.
This is similar to what we discussed yesterday about mitzvos. Some mitzvos are זמן גורם where the time frame is a factor in the very obligation of the mitzva [such as tztizis, tfillin and succah] and other mitzvos where the time frame is called זמן המגביל - just a "book-end" within which he must fulfill the mitzva [we said that women are only exempt from זמן גורם but not זמן המגביל and with that premise answered two questions of tosfos].
In the vernacular [you may use this on a date to impress a girl or if you are already married - your wife] - sometime the זמן is מעצם התנאי - part and parcel of the condition [the get case] while in other instances it is only הארכת זמן על הקיום - a time frame in which to perform the action [the house redeeming and neder fulfillment]. If it is מעצם התנאי then one is considered an אונס even for a second [because he is lacking a vital aspect of the תנאי - freedom of action during a certain time] while if it is just הארכת זמן then we say "Sorry dude - you shouldn't have pushed it off. You had time to do it earlier so how can we consider you אונס".
One very practical application of this concept is when one pushes off davening until the last minute and then when he is ready to daven - אונס strikes [such as a trip to the asher yatzar preparation room]. Is he considered אונס which would mandate two shmone esreis next time around or is he considered negligent for waiting? The Shulchan Aruch rules.... Awww - I can't tell you everything. I will let you find it yourself.
I must add that there is SOOOO much more to say on this topic but for now I am אנוס as Shabbos is slowly spreading her holy wings on Eretz Yisrael and I am compelled to spend the next 25 hours in bliss.... A close friend gave this topic more detailed treatmen here [the second article on the page].
לזכות ר' יעקב דב הכהן בן דוד לברכה והצלחה בכל מעשה ידיו