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diff --git a/subprojects/exess/src/timezone.h b/subprojects/exess/src/timezone.h
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+/*
+  Copyright 2021 David Robillard <d@drobilla.net>
+
+  Permission to use, copy, modify, and/or distribute this software for any
+  purpose with or without fee is hereby granted, provided that the above
+  copyright notice and this permission notice appear in all copies.
+
+  THIS SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#ifndef EXESS_TIMEZONE_H
+#define EXESS_TIMEZONE_H
+
+#include "exess/exess.h"
+
+#include <stddef.h>
+
+/// The maximum length of a canonical timezone string, 6
+#define EXESS_MAX_TIMEZONE_LENGTH 6
+
+/**
+   Read a timezone string after any leading whitespace.
+
+   @param out Set to the parsed value, or false on error.
+   @param str String input.
+   @return The `count` of characters read, and a `status` code.
+*/
+EXESS_API
+ExessResult
+exess_read_timezone(ExessTimezone* EXESS_NONNULL out,
+                    const char* EXESS_NONNULL    str);
+
+/**
+   Write a canonical timezone suffix.
+
+   The output is always in canonical form, either `Z` for UTC or a signed hour
+   and minute offset with leading zeros, like `-05:30` or `+14:00`.
+
+   @param value Value to write.
+   @param buf_size The size of `buf` in bytes.
+   @param buf Output buffer, or null to only measure.
+   @param o The current write offset in `buf`
+
+   @return #EXESS_SUCCESS on success, #EXESS_NO_SPACE if the buffer is too
+   small, or #EXESS_BAD_VALUE if the value is invalid.
+*/
+ExessResult
+write_timezone(ExessTimezone        value,
+               size_t               buf_size,
+               char* EXESS_NULLABLE buf,
+               size_t               o);
+
+#endif // EXESS_TIMEZONE_H