mu-utils: update casting from int64_t -> time_t

Esp. for systems with 32-bit time_t.

lib/utils/mu-utils.hh

@@ -36,7 +36,6 @@
 
 #include "mu-option.hh"
 
-
 #ifndef FMT_HEADER_ONLY
 #define FMT_HEADER_ONLY
 #endif /*FMT_HEADER_ONLY*/
@@ -268,6 +267,26 @@ static inline bool mu_print_encoded(fmt::format_string<T...> frm, T&&... args) n
 			     stdout);
 }
 
+/**
+ * Convert an int64_t to a time_t, clamping it within the range.
+ *
+ * This is only doing anything when using a 32-bit time_t value. This doesn't
+ * solve the 3038 problem, but at least allows for clearly marking where we
+ * convert
+ *
+ * @param t some 64-bit value that encodes a Unix time.
+ *
+ * @return a time_t value
+ */
+constexpr ::time_t time_t_min = 0;
+constexpr ::time_t time_t_max = std::numeric_limits<::time_t>::max();
+constexpr ::time_t to_time_t(int64_t t) {
+	return std::clamp(t,
+			  static_cast<int64_t>(time_t_min),
+			  static_cast<int64_t>(time_t_max));
+}
+
+
 /**
  * Parse a date string to the corresponding time_t
  * *