This calendar is based on the same idea as the atomic clock. Instead of the day, its base unit is the hour. It is designed so that there are 10000 hours in one year. In some respects it is similar to the Olympiad Thirds Calendar.

The new definition of the hour is:

The hour is defined by taking the fixed numerical value of the caesium frequency, Δ

ν_{Cs}, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 29009120000000 when expressed in the unit h^{−1}.

The main units that derive from this definition are:

Unit | Value | SI Unit Value | SI Unit Equivalent | Cs-133 transitions x1000 |
---|---|---|---|---|

hour | 1 h | 414416000000/131323311 s | 52 min 36 s | 29009120000 |

minute | 1/50 h | 8288320000/131323311 s | 63 s | 580182400 |

second | 1/50 min | 165766400/131323311 s | 1.26 s | 11603648 |

year | 10000 h | 365.242198 days | 290091200000000 | |

week | 1/50 year | 7.304844 days | 7 days 7 h 18 min 59 s | 5801824000000 |

day | 27 h 18 min 47.7317 s | 1 day | 794243384928 | |

31-day month | 850 h | 31.04 days | 31 days 1 h 5 min 38 s | 24657752000000 |

30-day month | 820 h | 29.95 days | 29 days 22 h 47 min 48 s | 23787478400000 |

28-day month | 770 h | 28.12 days | 28 days 2 h 58 min 3 s | 22337022400000 |

## Divisions of the year[]

The year is divided in twelve months that match the existing calendar.

January | February | March | April | May | June | July | August | September | October | November | December | |
---|---|---|---|---|---|---|---|---|---|---|---|---|

Length | 850 h | 770 h | 850 h | 820 h | 850 h | 820 h | 850 h | 850 h | 820 h | 850 h | 820 h | 850 h |

Days | 31 | 28 | 31 | 30 | 31 | 30 | 31 | 31 | 30 | 31 | 30 | 31 |

The year is divided in 50 weeks. An atomic week is a little more than seven days and it is not expected to displace the existing week days. It is mainly defined for timekeeping and administrative purposes.

The year can further be divided into two semesters of 25 weeks, five quintlods of 10 weeks or four quarters of 12.5 weeks.

## Partial days[]

The year is divided into 10000 hours, but it is not an exact number of days. That feature removes the need for leap days however it introduces the issue of partial days.

All of year, month and week don't align with an exact day. I propose the following rule:

If the boundary falls after 10 in the morning, then the transition is delayed until the next day.

Bear a mind that a day has 27 hours so 10 in the morning falls around 9am on ordinary clocks.

The rule means that an atomic month can gain or lose a day on occasion and an atomic week often spans eight instead of seven days. To anchor the calendar, 00:00 on 1 January 1903 is defined as the start of an atomic year. Due to both having Cs-133 transitions as base the day and the atomic calendar second align every 51802 days which means 00:00 on 29 October 2044 is also a round number (141 yr 8291 h 37 min 47 s in atomic calendar units).

## Time of the day[]

Since one atomic hour is worth about 53 minutes in SI units, each of the twelve divisions of the clock is about 1 h 7 min in atomic calendar time. For the minutes hand the divisions represent about four atomic minutes and same thing for the seconds hand.

It is more convenient to use a digital clock that can directly display the atomic calendar time.

A suitable value for noon is 14:00 which corresponds to about 12:15 on ordinary clocks. The time 1pm would correspond to 15:00 etc. An hypothetical atomic calendar clock face would not be able to match 1am with 1pm on the dial since 14:00 is not precisely the middle of the day, unless there is some mechanism to fast-forward the hours hand at the end of the day.

A more practical solution could be:

- Twenty-five divisions each representing 2 minutes or 2 seconds respectively or ten divisions representing 5 minutes or seconds each
- Hours in 27-hour format
- Notable hours marked with a number, but not with a precise position — the combination of the hours hand and minutes hand helps understand which is the actual hour