promotion rules for solar year and month

src/period.jl

@@ -96,6 +96,16 @@ function promote_rule(::Type{Period{T1,Tfactor1,Texponent1}},
     end
 end
 
+for Tfactor1 in (SOLAR_YEAR, SOLAR_YEAR ÷ 12)
+    @eval function promote_rule(::Type{Period{T1,Val($Tfactor1), Val(-3)}},
+                                ::Type{Period{T2,Tfactor2,Texponent2}}) where
+        {T1,T2,Tfactor2,Texponent2}
+        return promote_rule(
+            Period{T1,Val(1), Val(-3)},
+            Period{T2,Tfactor2,Texponent2})
+    end
+end
+
 
 function convert(::Type{Period{T1,Tfactor1,Texponent1}},
                  p::Period{T2,Tfactor2,Texponent2}) where

src/query.jl

@@ -6,6 +6,7 @@
 @inline _hasyear0(::Type{T}) where T <: AbstractCFDateTime = true
 
 
+
 """
     monthlength = daysinmonth(::Type{DT},y,m)
 

test/test_resolution.jl

@@ -328,5 +328,8 @@ end
 @test CFTime.datetuple(CFTime.timedecode(0,"days since -4713-01-01T12:00:00","julian", prefer_datetime = false)) ==
     (-4713, 1, 1, 12, 0, 0, 0)
 
+dt = CFTime.timedecode([0,1],"years since 2000-01-01T00:00:00", prefer_datetime = false)
+@test Dates.value(Dates.Millisecond(dt[2] - dt[1])) == CFTime.SOLAR_YEAR
 
-
+dt = CFTime.timedecode([0,1],"months since 2000-01-01T00:00:00", prefer_datetime = false)
+@test Dates.value(Dates.Millisecond(dt[2] - dt[1])) == CFTime.SOLAR_YEAR ÷ 12

test/test_year0.jl

@@ -33,3 +33,13 @@ dt = CFTime.timedecode(-1,"days since 0001-01-01",prefer_datetime = false)
 @test dt == DateTimeStandard(-1,12,31)
 @test CFTime.year(dt) == -1
 @test CFTime.month(dt) == 12
+
+# test a dummy calendar with no year 0 and regular month length
+struct DummyDataTime{T,Torigintuple} <: CFTime.AbstractCFDateTime{T,Torigintuple}
+end
+import CFTime: _cum_month_length, _hasyear0
+_cum_month_length(::Type{DummyDataTime}) = (0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330, 360)
+_hasyear0(::Type{DummyDataTime}) = false
+
+Z = datenum(DummyDataTime, -1, 1, 1)
+@test datetuple_ymd(DummyDataTime,Z) == (-1,1,1)