[bugfix] Parse scheduled_at as ISO8601 with offset if RFC3339 parse fails (#4431)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4409 by reattempting `scheduled_at` parsing using ISO8601 offset.

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4431
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

internal/api/client/statuses/statuscreate.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"time"
 
 	apimodel "code.superseriousbusiness.org/gotosocial/internal/api/model"
 	apiutil "code.superseriousbusiness.org/gotosocial/internal/api/util"
@@ -259,8 +260,6 @@ import (
 //			description: unprocessable content
 //		'500':
 //			description: internal server error
-//		'501':
-//			description: scheduled_at was set, but this feature is not yet implemented
 func (m *Module) StatusCreatePOSTHandler(c *gin.Context) {
 	authed, errWithCode := apiutil.TokenAuth(c,
 		true, true, true, true,
@@ -414,5 +413,30 @@ func parseStatusCreateForm(c *gin.Context) (*apimodel.StatusCreateRequest, gtser
 		form.Poll.ExpiresIn = util.PtrOrZero(expiresIn)
 	}
 
+	// Parse scheduled_at if given.
+	if form.ScheduledAtRaw != "" {
+		// Try RFC3339 initially, which
+		// is a stricter UTC subset of ISO8601.
+		scheduledAt, err := time.Parse(
+			time.RFC3339, form.ScheduledAtRaw,
+		)
+		if err != nil {
+			// Try ISO8601 with offset
+			// (many clients use this).
+			scheduledAt, err = time.Parse(
+				util.ISO8601Offset, form.ScheduledAtRaw,
+			)
+		}
+
+		// If we still have an error
+		// we can't use this time.
+		if err != nil {
+			text := "could not parse scheduled_at value " + form.ScheduledAtRaw + " as ISO8601 time"
+			return nil, gtserror.NewErrorBadRequest(errors.New(text), text)
+		}
+
+		form.ScheduledAt = &scheduledAt
+	}
+
 	return form, nil
 }