If you hover your mouse over the time marked in the right-bottom corner of this post, then you will see a string showing the exact time that I post this question, which reads 2021-03-18 22:32:47Z. See this post for more details on how to check the exact time of posting.

Ignoring all non-digit characters, that time gives the number $20210318223247$. It turns out that this number has exactly $4$ divisors.

Now I offer a bounty of 500 reputations to reward any answer to this question whose posting time gives a number that has exactly $5$ divisors.

When should you post your answer?

Note: If you post a correct answer at a time that does not meet the above criterion, the answer will still be accepted. You just don't get the bounty.

  • $\begingroup$ FYI: At least on my browser - Chrome on a desktop, the time appears in the top-left, not the bottom-right. Is it different for the mobile version? $\endgroup$ Mar 19, 2021 at 13:35
  • 1
    $\begingroup$ @DarrelHoffman: It's in both places, you can see it above the username in the bottom-right of the question too. Hover the "asked 16 hours ago" label (of whatever it may say at time of reading this). $\endgroup$
    – musefan
    Mar 19, 2021 at 15:31
  • $\begingroup$ Whoa, what? You can view the timestamp? I've been on Stack Exchange for over 8 years and this is the first time I've heard of this feature, lol I feel dumb... $\endgroup$
    – QBrute
    Mar 21, 2021 at 0:35

3 Answers 3


I'm a very impatient person so I've decided not to wait around to post an answer at the right time which I think will be

7461-02-24 07:38:41Z


Numbers which have exactly five divisors are those of the form $p^4$ where $p$ is a prime so it is a matter of searching through fourth powers of primes which look like a date and time (some get quite close).

  • 7
    $\begingroup$ Good job! Don't forget to come back for the bounty when it's the right time ^_^ $\endgroup$
    – WhatsUp
    Mar 18, 2021 at 23:34
  • 2
    $\begingroup$ No one wants to wait around for 918 years just for 500 rep points. So I am writing a script to do it... $\endgroup$ Mar 20, 2021 at 3:56

Having 5 divisors means

that the number is exactly the fourth power of a prime number.

A standard number-theoretical way to observe this is

to look at the properties of Number of Divisors function $\sigma_0(n)$, specifically $$ \sigma_0(p_1^{e_1}p_2^{e_2}\cdots) = (1+e_1)(1+e_2)\cdots $$ Since this value is composite when $n$ has at least two distinct prime factors, and 5 is prime, the only way to satisfy the right side being 5 is that $n$ has a single prime factor $p_1$ and $e_1=4$, i.e. $n = p^4$.

So what we need to do now is

write a program that goes through all primes after $^4\sqrt {20210318223247}$ and find the first one whose fourth power is a valid timestamp (has valid month, day, hour, minute, second).

Here is the code in Factor:

20210318223247 0.25 ^ 1 /i [ dup 4 ^
  { [ 10 8 ^ / floor 100 mod 1 12 between? ]
    [ 10 6 ^ / floor 100 mod 1 31 between? ]
    [ 10000 / floor 100 mod 24 < ]
    [ 100 / floor 100 mod 60 < ]
    [ 100 mod 60 < ] } 1&&
] [ next-prime ] do until dup . 4 ^ .

Try it online!

This code does not very rigorously check the validity of a timestamp, in that something like Feb 31 is not filtered out. Luckily, the first answer found has a well-formed month-day combination.

The output is

$2939^4 = 74610224073841$, which maps to 7461-02-24 07:38:41Z. It's way too distant in the future for us to wait for that time, as hexomino already pointed out :)

  • $\begingroup$ Ah ... very well-written answer! However you are just a tiiiiny bit slower than hexomino. I hope you understand that I give the green check to hexomino. Of course you still get my upvote! $\endgroup$
    – WhatsUp
    Mar 18, 2021 at 23:36

You need to post your answer on:

A time that is the fourth power of a prime number. The other answers have already posted the first one that works.

The way to find this is:

First, notice that it is somewhat difficult to have a number $n$ with an odd number of divisors. If $j$ is a divisor of $n$, then $\frac{n}{j} = k$ is also a divisor. The only way for divisors to not come in pairs is if you have a square number so that $j=k$. Thus our desired $n$ must be equal to the square of some number $m$. $m$ cannot be prime, otherwise $n$ would only have 3 factors. So $m$ has a factor $j$, meaning that $j$ and $mj$ are also factors of $n$. Thus the factors of $n$ are 1, $j$, $m$, $mj$, and $n$, so $j$ must be a prime number and $n=j^4$.

To make it even harder:

Most primes still won't work. For example, $2129^4=20544834434881$, but unless there are some significant changes in the next 30 years there will not be a 34th day of the 48th month in 2054.

  • 1
    $\begingroup$ Notice that there is a computer-puzzle tag, which means that it is intended to be solved using a computer. $\endgroup$
    – WhatsUp
    Mar 18, 2021 at 23:40
  • $\begingroup$ @WhatsUp I was writing up my explanation of why the number need to have the property that it does when Hexomino answered. I decided not to bother finishing my program to find it because I was already beaten to the punch. $\endgroup$
    – Rob Watts
    Mar 18, 2021 at 23:45
  • $\begingroup$ Oh yes, I appreciate your detailed explanation (+1). $\endgroup$
    – WhatsUp
    Mar 18, 2021 at 23:49

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