Alexander Belopolsky added the comment:
The following code demonstrates that we can pack year through second fields in
40 bits:
#include <stdio.h>
struct dt {
unsigned year :14; /* 1-9999 of 0-16,383 */
unsigned month :4; /* 1-12 of 0-16 */
unsigned day :5; /* 1-31 of 0-31 */
unsigned hour :5; /* 0-23 of 0-31 */
unsigned minute:6; /* 0-59 of 0-63 */
unsigned second:6; /* 0-59 of 0-63 */
/* total : 40 bits */
};
int main() {
struct dt t;
t.year = 9999;
t.month = 12;
t.day = 31;
t.hour = 24;
t.minute = 59;
t.second = 59;
printf("%d-%d-%dT%d:%d:%d in %d bytes\n",
t.year, t.month, t.day,
t.hour, t.minute, t.second,
(int)sizeof(t));
}
$ ./a.out
9999-12-31T24:59:59 in 8 bytes
Assuming 64-bit alignment, this leaves 64*2 - 40 = 88 bits for the sub-second
field.
In 88 bits you can pack yoctoseconds (yocto = 1e-24) without breaking a sweat:
>>> 2**88
309485009821345068724781056
>>> 10**24
1000000000000000000000000
The practical choice is between 32-bit nanoseconds (nano = 1e-9) and 64-bit
attoseconds (atto = 1e-18).
Given that the current the world record for shortest controllable time is 12
attoseconds [1], it may not be an absurd choice to go to 64 bits of sub-second
precision.
On the other hand, if we manage to go from micro- to nano-seconds now, I don't
think it will be hard to go to higher resolution when users start asking for it
some 40 years into the future.
[1] http://phys.org/news192909576.html
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