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Businesses sometimes use leap week calendars for bookkeeping, but differ in the format used. These formats usually divide the year into equal quarters of 91 days each, but within those quarters often differ between using nearly equal months of 31-30-30, 30-31-30, or 30-30-31 days, or using whole weeks of 5-4-4, 4-5-4 or 4-4-5 weeks.

Assuming an ISO weekday system is used, with ISO W53 being the same leap week in all cases, there are thus 6 possible formats. These formats can have the same start date for each quarter, but different month lengths for each of the 6 possible formats. These 6 possible formats can be given unique month names which thus allow all 6 of them to be used simultaneously and unambiguously. The following is an example of such a month naming scheme:

Systematic month names
Week 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
13mon Janoon Febroon Marchoon Aproon Mayoon Junioon Extroon Juloon Augoon Septoon Octoon Novoon Decoon
5:4:4 Janaire Febraire Marchose Aprilose Mayose Junal Julial Augial Septidor Octidor Novidor Decaire
4:5:4 Janose Febrose Aprial Mayal Julidor Augidor Octaire Novaire
4:4:5 Febrose Marchal Mayal Junidor Augidor Septaire Novaire Decose
31:30:30 Janum Febra Marcha Aprilum Maya Juna Julium Auga Septa Octum Nova Deca
30:31:30 Janum Februm Aprilum Mayum Julium Augum Octum Novum
30:30:31 Februm Marchum Mayum Junum Augum Septum Novum Decum
Q Quartose Quartial Quartidor Quartaire
First month per quarter longer
Month 31-30-30 Days Start Month 5-4-4 Days Start
Janum (Jam) 31 W01-1 Janaire (Jar) 35 W01-1
Febra (Fba) 30 W05-4 Febraire (Fbr) 28 W06-1
Marcha (Mra) 30 W09-6 Marchose (Mro) 28 W10-1
Aprilum (Apm) 31 W14-1 Apriose (Apo) 35 W14-1
Maya (Mya) 30 W18-4 Mayose (Myo) 28 W19-1
Juna (Jna) 30 W22-6 Junal (Jnl) 28 W23-1
Julium (Jlm) 31 W27-1 Julial (Jll) 35 W27-1
Auga (Aua) 30 W31-4 Augial (Aul) 28 W32-1
Septa (Spa) 30 W35-6 Septidor (Spd) 28 W36-1
Octum (Ocm) 31 W40-1 Octidor (Ocd) 35 W40-1
Nova (Nva) 30 W44-4 Novidor (Nvd) 28 W45-1
Deca (Dca) 30/37 W48-6 Decaire (Dcr) 28/35 W49-1
Middle month per quarter longer
Month 30-31-30 Days Start Month 4-5-4 Days Start
Janum (Jam) 30 W01-1 Janose (Jao) 28 W01-1
Februm (Fbm) 31 W05-3 Febrose (Fbo) 35 W05-1
Marcha (Mra) 30 W09-6 Marchose (Mro) 28 W10-1
Aprilum (Apm) 30 W14-1 Aprial (Apl) 28 W14-1
Mayum (Mym) 31 W18-3 Mayal (Myl) 35 W18-1
Juna (Jna) 30 W22-6 Junal (Jnl) 28 W23-1
Julium (Jlm) 30 W27-1 Julidor (Jld) 28 W27-1
Augum (Aum) 31 W31-3 Augidor (Aud) 35 W31-1
Septa (Spa) 30 W35-6 Septidor (Spd) 28 W36-1
Octum (Ocm) 30 W40-1 Octaire (Ocr) 28 W40-1
Novum (Nvm) 31 W44-3 Novaire (Nvr) 35 W44-1
Deca (Dca) 30/37 W48-6 Decaire (Dcr) 28/35 W49-1
Last month per quarter longer
Month 30-30-31 Days Start Month name 4-4-5 Days Start
Janum (Jam) 30 W01-1 Janose (Jao) 28 W01-1
Februm (Fbm) 30 W05-3 Febrose Febral (Fbo) 28 W05-1
Marchum (Mrm) 31 W09-5 Marchal (Mrl) 35 W09-1
Aprilum (Apm) 30 W14-1 Aprial (Apl) 28 W14-1
Mayum (Mym) 30 W18-3 Mayal Mayidor (Myl) 28 W18-1
Junum (Jnm) 31 W22-5 Junidor (Jnd) 35 W22-1
Julium (Jlm) 30 W27-1 Julidor (Jld) 28 W27-1
Augum (Aum) 30 W31-3 Augidor Augaire (Aud) 28 W31-1
Septum (Spm) 31 W35-5 Septaire (Spr) 35 W35-1
Octum (Ocm) 30 W40-1 Octaire (Ocr) 28 W40-1
Novum (Nvm) 30 W44-3 Novaire Novose (Nvr) 28 W44-1
Decum (Dcm) 31/38 W48-5 Decose (Dcr) 35/42 W48-1
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