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, ΔνCs, 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:

Table of units
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.

Months of the year
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