ARROW-13734: [Format] Clarify allowed values for time types

format/Schema.fbs

@@ -194,8 +194,8 @@ enum DateUnit: short {
   MILLISECOND
 }
 
-/// Date is either a 32-bit or 64-bit type representing elapsed time since UNIX
-/// epoch (1970-01-01), stored in either of two units:
+/// Date is either a 32-bit or 64-bit signed integer type representing an
+/// elapsed time since UNIX epoch (1970-01-01), stored in either of two units:
 ///
 /// * Milliseconds (64 bits) indicating UNIX time elapsed since the epoch (no
 ///   leap seconds), where the values are evenly divisible by 86400000
@@ -206,9 +206,20 @@ table Date {
 
 enum TimeUnit: short { SECOND, MILLISECOND, MICROSECOND, NANOSECOND }
 
-/// Time type. The physical storage type depends on the unit
-/// - SECOND and MILLISECOND: 32 bits
-/// - MICROSECOND and NANOSECOND: 64 bits
+/// Time is either a 32-bit or 64-bit signed integer type representing an
+/// elapsed time since midnight, stored in either of four units: seconds,
+/// milliseconds, microseconds or nanoseconds.
+///
+/// The integer `bitWidth` depends on the `unit` and must be one of the following:
+/// * SECOND and MILLISECOND: 32 bits
+/// * MICROSECOND and NANOSECOND: 64 bits
+///
+/// The allowed values are between 0 (inclusive) and 86400 (=24*60*60) seconds
+/// (exclusive), adjusted for the time unit (for example, up to 86400000
+/// exclusive for the MILLISECOND unit).
+/// This definition doesn't allow for leap seconds. Time values from
+/// measurements with leap seconds will need to be corrected when ingesting
+/// into Arrow (for example by replacing the value 86400 with 86399).
 table Time {
   unit: TimeUnit = MILLISECOND;
   bitWidth: int = 32;