| Unit | Symbol | Definition | Conversion to SI units |
| atomic unit of time |
au |
= a0/(a·c) |
~ 2.418 884 254 x 10-17 s |
| Callippic cycle |
|
= 441 mo (hollow) + 499 mo (full) = 76 a of 365.25 d |
= 2.398 3776 x 109 s |
| century |
c |
= 100 a (see below for definition of year length) |
= 100 x year |
| day |
d |
= 24 h |
= 86400 s |
| day (sidereal) |
d |
= Time needed for the Earth to rotate once around its axis, determined from successive transits of a very distant astronomical object across an observer's meridian (International Celestial Reference Frame) |
~ 86 164.1 s |
| decade |
dec |
= 10 a (see below for definition of year length) |
= 10 x year |
| fortnight |
fn |
= 2 wk |
= 1 209 600 s |
| helek |
|
= 1/1 080 h |
= 3.3 s |
| Hipparchic cycle |
|
= 4 Callippic cycles - 1 d |
= 9.593 424 x 109 s |
| hour |
h |
= 60 min |
= 3 600 s |
| jiffy |
j |
= 1/60 s |
= .016 s |
| jiffy (alternate) |
ja |
= 1/100 s |
= 10 ms |
| ke (quarter of an hour) |
|
= ¼ h = 1/96 d |
= 60 x 60 / 4 s = 900 s = 60 / 4 min = 15 min |
| ke (traditional) |
|
= 1/100 d |
= 24 x 60 x 60 / 100 s = 864 s = 24 * 60 / 100 min = 14.4 min |
| lustre; lustrum |
|
= 5 a of 365 d |
= 1.5768 x 108 s |
| Metonic cycle; enneadecaeteris |
|
= 110 mo (hollow) + 125 mo (full) = 6940 d ~ 19 a |
= 5.996 16 x 108 s |
| millennium |
|
= 1 000 a (see below for definition of year length) |
= 1000 x year |
| milliday |
md |
= 1/1 000 d |
= 24 x 60 x 60 / 1 000 s = 86.4 s |
| minute |
min |
= 60 s, due to leap seconds sometimes 59 s or 61 s, |
= 60 s |
| moment |
|
= 90 s |
= 90 s |
| month (full) |
mo |
= 30 d |
= 2 592 000 s |
| month (Greg. av.) |
mo |
Average Gregorian month = 365.2425/12 d = 30.436875 d |
~ 2.6297 x 106 s |
| month (hollow) |
mo |
= 29 d |
= 2 505 600 s |
| month (synodic) |
mo |
Cycle time of moon phases ~ 29.530589 days (Average) |
~ 2.551 x 106 s |
| octaeteris |
|
= 48 mo (full) + 48 mo (hollow) + 3 mo (full) = 8 a of 365.25 d = 2922 d |
= 2.524 608 x 108 s |
| Planck time |
|
= (G?/c5)½ |
~ 1.351 211 868 x 10-43 s |
| second |
s |
time of 9 192 631 770 periods of the radiation corresponding to the transition between the 2 hyperfine levels of the ground state of the caesium 133 atom at 0 K (but other seconds are sometimes used in astronomy) |
(SI base unit) |
| shake |
|
= 10-8 s |
= 10 ns |
| sigma |
|
= 10-6 s |
= 1 µs |
| Sothic cycle |
|
= 1 461 a of 365 d |
= 4.607 4096 x 1010 s |
| svedberg |
S |
= 10-13 s |
= 100 fs |
| week |
wk |
= 7 d |
= 604 800 s |
| year (Gregorian) |
a, y, or yr |
= 365.2425 d average, calculated from common years (365 d) plus leap years (366 d) on most years divisible by 4. See leap year for details. |
= 31 556 952 s |
| year (Julian) |
a, y, or yr |
= 365.25 d average, calculated from common years (365 d) plus one leap year (366 d) every four years |
= 31 557 600 s |
| year (sidereal) |
a, y, or yr |
= time taken for Sun to return to the same position with respect to the stars of the celestial sphere |
~ 365.256 363 d ~ 31 558 149.7632 s |
| year (tropical) |
a, y, or yr |
= Length of time it takes for the Sun to return to the same position in the cycle of seasons |
~ 365.242 190 d ~ 31 556 925 s |
| Where UTC is observed, the length of time units longer than 1 s may increase or decrease by 1 s if a leap second occurs during the time interval of interest. |
