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fix(js/ts): date arithmetic: PM parsing, AddMonths day clamp, AddYears offset #4744

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Merged
MangelMaxime merged 2 commits into from
Jul 10, 2026
+48 −30
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fix(js/ts): date arithmetic: PM parsing, AddMonths day clamp, AddYears offset #4744

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9d80f49
fix: date arithmetic: PM parsing, AddMonths day clamp, AddYears offset
Thorium Jul 8, 2026
6060ace
chore: remove comment
MangelMaxime Jul 10, 2026
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19 changes: 6 additions & 13 deletions src/fable-library-ts/Date.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -636,7 +636,9 @@ export function parseRaw(input: string): [Date, Offset] {
parseInt(timeParts[1] || "0", 10) * 60 +
parseFloat(timeParts[2] || "0");
if (m[3] != null && m[3].toUpperCase() === "PM" && hourPart < 12) {
timeInSeconds += 720;
timeInSeconds += 12 * 3600;
} else if (m[3] != null && m[3].toUpperCase() === "AM" && hourPart === 12) {
timeInSeconds -= 12 * 3600;
}
}
if (m[4] != null) { // There's an offset, parse as UTC
Expand Down Expand Up @@ -854,18 +856,9 @@ export function addYears(d: IDateTime, v: number) {
}

export function addMonths(d: IDateTime, v: number) {
let newMonth = month(d) + v;
let newMonth_ = 0;
let yearOffset = 0;
if (newMonth > 12) {
newMonth_ = newMonth % 12;
yearOffset = Math.floor(newMonth / 12);
newMonth = newMonth_;
} else if (newMonth < 1) {
newMonth_ = 12 + newMonth % 12;
yearOffset = Math.floor(newMonth / 12) + (newMonth_ === 12 ? -1 : 0);
newMonth = newMonth_;
}
const totalMonths = month(d) + v;
const newMonth = ((totalMonths - 1) % 12 + 12) % 12 + 1;
const yearOffset = Math.floor((totalMonths - 1) / 12);
const newYear = year(d) + yearOffset;
const _daysInMonth = daysInMonth(newYear, newMonth);
const newDay = Math.min(_daysInMonth, day(d));
Expand Down
26 changes: 9 additions & 17 deletions src/fable-library-ts/DateOffset.ts
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Expand Up @@ -249,28 +249,20 @@ export function addTicks(d: IDateTimeOffset, v: int64) {
}

export function addYears(d: IDateTimeOffset, v: number) {
const newMonth = d.getUTCMonth() + 1;
const newYear = d.getUTCFullYear() + v;
const d2 = new Date(d.getTime() + offset(d));
const newMonth = d2.getUTCMonth() + 1;
const newYear = d2.getUTCFullYear() + v;
const _daysInMonth = daysInMonth(newYear, newMonth);
const newDay = Math.min(_daysInMonth, d.getUTCDate());
return create(newYear, newMonth, newDay, d.getUTCHours(), d.getUTCMinutes(),
d.getUTCSeconds(), d.getUTCMilliseconds(), offset(d));
const newDay = Math.min(_daysInMonth, d2.getUTCDate());
return create(newYear, newMonth, newDay, d2.getUTCHours(), d2.getUTCMinutes(),
d2.getUTCSeconds(), d2.getUTCMilliseconds(), offset(d));
}

export function addMonths(d: IDateTimeOffset, v: number) {
const d2 = new Date(d.getTime() + offset(d));
let newMonth = d2.getUTCMonth() + 1 + v;
let newMonth_ = 0;
let yearOffset = 0;
if (newMonth > 12) {
newMonth_ = newMonth % 12;
yearOffset = Math.floor(newMonth / 12);
newMonth = newMonth_;
} else if (newMonth < 1) {
newMonth_ = 12 + newMonth % 12;
yearOffset = Math.floor(newMonth / 12) + (newMonth_ === 12 ? -1 : 0);
newMonth = newMonth_;
}
const totalMonths = d2.getUTCMonth() + 1 + v;
const newMonth = ((totalMonths - 1) % 12 + 12) % 12 + 1;
const yearOffset = Math.floor((totalMonths - 1) / 12);
const newYear = d2.getUTCFullYear() + yearOffset;
const _daysInMonth = daysInMonth(newYear, newMonth);
const newDay = Math.min(_daysInMonth, d2.getUTCDate());
Expand Down
14 changes: 14 additions & 0 deletions tests/Js/Main/DateTimeOffsetTests.fs
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Expand Up @@ -412,6 +412,20 @@
test -20 2050
test -100 2046

testCase "DateTimeOffset.AddMonths keeps last day when delta is a multiple of 12" <| fun () ->
let dt = DateTimeOffset(2020, 12, 31, 0, 0, 0, TimeSpan.Zero).AddMonths(12)
dt.Year |> equal 2021
dt.Month |> equal 12
dt.Day |> equal 31

testCase "DateTimeOffset.AddYears keeps offset-local wall clock" <| fun () ->
let dt = DateTimeOffset(2020, 1, 1, 2, 0, 0, TimeSpan.FromHours 5.).AddYears(1)
dt.Year |> equal 2021
dt.Month |> equal 1
dt.Day |> equal 1
dt.Hour |> equal 2
dt.Offset |> equal (TimeSpan.FromHours 5.)

testCase "DateTimeOffset.AddDays works" <| fun () ->
let test v expected =
let dt = DateTimeOffset(2014,9,12,0,0,0,TimeSpan.Zero).AddDays(v)
Expand Down Expand Up @@ -888,7 +902,7 @@

testCase "Implicit conversion from DateTime preserves value" <| fun () ->
let d = DateTime(2024, 1, 15, 0, 0, 0, DateTimeKind.Utc)
let dto: DateTimeOffset = d

Check warning on line 905 in tests/Js/Main/DateTimeOffsetTests.fs

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GitHub Actions / build-javascript (ubuntu-latest) 

This expression uses the implicit conversion 'DateTimeOffset.op_Implicit(dateTime: DateTime) : DateTimeOffset' to convert type 'DateTime' to type 'DateTimeOffset'. See https://aka.ms/fsharp-implicit-convs. This warning may be disabled using '#nowarn "3391".

Check warning on line 905 in tests/Js/Main/DateTimeOffsetTests.fs

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GitHub Actions / build-javascript (windows-latest) 

This expression uses the implicit conversion 'DateTimeOffset.op_Implicit(dateTime: DateTime) : DateTimeOffset' to convert type 'DateTime' to type 'DateTimeOffset'. See https://aka.ms/fsharp-implicit-convs. This warning may be disabled using '#nowarn "3391".
dto.UtcDateTime |> equal d
]
]
19 changes: 19 additions & 0 deletions tests/Js/Main/DateTimeTests.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -848,6 +848,19 @@ let tests =
d3.Hour + d3.Minute + d3.Second |> equal 54


testCase "DateTime.Parse handles AM/PM designator correctly" <| fun () ->
// Time-only strings parse to clock fields that are timezone-independent
let d = DateTime.Parse("1:05:34 PM", CultureInfo.InvariantCulture)
d.Hour |> equal 13
d.Minute |> equal 5
d.Second |> equal 34
let d = DateTime.Parse("12:05 AM", CultureInfo.InvariantCulture)
d.Hour |> equal 0
d.Minute |> equal 5
let d = DateTime.Parse("12:05 PM", CultureInfo.InvariantCulture)
d.Hour |> equal 12
d.Minute |> equal 5

testCase "DateTime.TryParse works" <| fun () ->
let (isSuccess, _) = DateTime.TryParse("foo", CultureInfo.InvariantCulture, DateTimeStyles.None)
isSuccess |> equal false
Expand Down Expand Up @@ -1005,6 +1018,12 @@ let tests =
test -20 2050
test -100 2046

testCase "DateTime.AddMonths keeps last day when delta is a multiple of 12" <| fun () ->
let dt = DateTime(2020, 12, 31, 0, 0, 0, DateTimeKind.Utc).AddMonths(12)
dt.Year |> equal 2021
dt.Month |> equal 12
dt.Day |> equal 31

testCase "DateTime.AddDays works" <| fun () ->
let test v expected =
let dt = DateTime(2014,9,12,0,0,0,DateTimeKind.Utc).AddDays(v)
Expand Down
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