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diff --git a/subprojects/exess/src/date_utils.h b/subprojects/exess/src/date_utils.h
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+++ b/subprojects/exess/src/date_utils.h
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+/*
+  Copyright 2019-2021 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_DATE_UTILS_H
+#define EXESS_DATE_UTILS_H
+
+#include "exess/exess.h"
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+static inline bool
+is_leap_year(const int64_t year)
+{
+  if (year % 4) {
+    return false;
+  }
+
+  if (year % 100) {
+    return true;
+  }
+
+  if (year % 400) {
+    return false;
+  }
+
+  return true;
+}
+
+static inline uint8_t
+days_in_month(const int16_t year, const uint8_t month)
+{
+  return month == 2u ? (is_leap_year(year) ? 29u : 28u)
+                     : (uint8_t)(30u + (month + (month / 8u)) % 2u);
+}
+
+ExessResult
+read_year_number(int16_t* out, const char* str);
+
+ExessResult
+write_year_number(int16_t value, size_t buf_size, char* buf);
+
+/// Read YYYY-MM-DD date numbers without a timezone
+ExessResult
+read_date_numbers(ExessDate* out, const char* str);
+
+EXESS_CONST_FUNC
+size_t
+exess_timezone_string_length(ExessTimezone value);
+
+#endif // EXESS_DATE_UTILS_H