Add SetDateTimeValue which makes sure the formatting will be as specified in the spec

src/ImageSharp/Metadata/Profiles/IPTC/IptcProfile.cs

@@ -174,6 +174,7 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Iptc
         public void SetValue(IptcTag tag, Encoding encoding, string value, bool strict = true)
         {
             Guard.NotNull(encoding, nameof(encoding));
+            Guard.NotNull(value, nameof(value));
 
             if (!tag.IsRepeatable())
             {
@@ -192,6 +193,27 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Iptc
             this.values.Add(new IptcValue(tag, encoding, value, strict));
         }
 
+        /// <summary>
+        /// Makes sure the datetime is formatted according to the iptc specification.
+        /// A date will be formatted as CCYYMMDD.
+        /// A time value will be formatted as HHMMSS±HHMM.
+        /// </summary>
+        /// <param name="tag">The tag of the iptc value.</param>
+        /// <param name="dateTime">The datetime.</param>
+        public void SetDateTimeValue(IptcTag tag, DateTime dateTime)
+        {
+            if (!tag.IsDate() && !tag.IsTime())
+            {
+                throw new ArgumentException("iptc tag is not a time or date type");
+            }
+
+            var formattedDate = tag.IsDate()
+                ? dateTime.ToString("yyyyMMdd", System.Globalization.CultureInfo.InvariantCulture)
+                : dateTime.ToString("HHmmsszzzz", System.Globalization.CultureInfo.InvariantCulture).Replace(":", string.Empty);
+
+            this.SetValue(tag, Encoding.UTF8, formattedDate);
+        }
+
         /// <summary>
         /// Sets the value of the specified tag.
         /// </summary>

src/ImageSharp/Metadata/Profiles/IPTC/IptcTag.cs

@@ -86,30 +86,28 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Iptc
 
         /// <summary>
         /// Release date. Format should be CCYYMMDD,
-        /// e.g. "19890317" indicates data for release on 17 March 1989.
+        /// e.g. "19890317" for the 17 March 1989.
         /// Not repeatable, max length is 8.
         /// </summary>
         ReleaseDate = 30,
 
         /// <summary>
         /// Release time. Format should be HHMMSS±HHMM,
-        /// e.g. "090000-0500" indicates object for use after 0900 in
-        /// New York (five hours behind UTC)
+        /// e.g. "090000-0500" for 9 o'clock New York time (five hours behind UTC).
         /// Not repeatable, max length is 11.
         /// </summary>
         ReleaseTime = 35,
 
         /// <summary>
         /// Expiration date. Format should be CCYYMMDD,
-        /// e.g. "19890317" indicates data for release on 17 March 1989.
+        /// e.g. "19890317" for the 17 March 1989.
         /// Not repeatable, max length is 8.
         /// </summary>
         ExpirationDate = 37,
 
         /// <summary>
         /// Expiration time. Format should be HHMMSS±HHMM,
-        /// e.g. "090000-0500" indicates object for use after 0900 in
-        /// New York (five hours behind UTC)
+        /// e.g. "090000-0500" for 9 o'clock New York time (five hours behind UTC).
         /// Not repeatable, max length is 11.
         /// </summary>
         ExpirationTime = 38,
@@ -131,7 +129,7 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Iptc
 
         /// <summary>
         /// Reference date. Format should be CCYYMMDD,
-        /// e.g. "19890317" indicates data for release on 17 March 1989.
+        /// e.g. "19890317" for the 17 March 1989.
         /// Not repeatable, max length is 8.
         /// </summary>
         ReferenceDate = 47,
@@ -143,30 +141,28 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Iptc
 
         /// <summary>
         /// Created date. Format should be CCYYMMDD,
-        /// e.g. "19890317" indicates data for release on 17 March 1989.
+        /// e.g. "19890317" for the 17 March 1989.
         /// Not repeatable, max length is 8.
         /// </summary>
         CreatedDate = 55,
 
         /// <summary>
         /// Created time. Format should be HHMMSS±HHMM,
-        /// e.g. "090000-0500" indicates object for use after 0900 in
-        /// New York (five hours behind UTC)
+        /// e.g. "090000-0500" for 9 o'clock New York time (five hours behind UTC).
         /// Not repeatable, max length is 11.
         /// </summary>
         CreatedTime = 60,
 
         /// <summary>
         /// Digital creation date. Format should be CCYYMMDD,
-        /// e.g. "19890317" indicates data for release on 17 March 1989.
+        /// e.g. "19890317" for the 17 March 1989.
         /// Not repeatable, max length is 8.
         /// </summary>
         DigitalCreationDate = 62,
 
         /// <summary>
         /// Digital creation time. Format should be HHMMSS±HHMM,
-        /// e.g. "090000-0500" indicates object for use after 0900 in
-        /// New York (five hours behind UTC)
+        /// e.g. "090000-0500" for 9 o'clock New York time (five hours behind UTC).
         /// Not repeatable, max length is 11.
         /// </summary>
         DigitalCreationTime = 63,

src/ImageSharp/Metadata/Profiles/IPTC/IptcTagExtensions.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 namespace SixLabors.ImageSharp.Metadata.Profiles.Iptc
@@ -117,5 +117,46 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Iptc
                     return true;
             }
         }
+
+        /// <summary>
+        /// Determines if the tag is a datetime tag which needs to be formatted as CCYYMMDD.
+        /// </summary>
+        /// <param name="tag">The tag to check.</param>
+        /// <returns>True, if its a datetime tag.</returns>
+        public static bool IsDate(this IptcTag tag)
+        {
+            switch (tag)
+            {
+                case IptcTag.CreatedDate:
+                case IptcTag.DigitalCreationDate:
+                case IptcTag.ExpirationDate:
+                case IptcTag.ReferenceDate:
+                case IptcTag.ReleaseDate:
+                    return true;
+
+                default:
+                    return false;
+            }
+        }
+
+        /// <summary>
+        /// Determines if the tag is a time tag which need to be formatted as HHMMSS±HHMM.
+        /// </summary>
+        /// <param name="tag">The tag to check.</param>
+        /// <returns>True, if its a time tag.</returns>
+        public static bool IsTime(this IptcTag tag)
+        {
+            switch (tag)
+            {
+                case IptcTag.CreatedTime:
+                case IptcTag.DigitalCreationTime:
+                case IptcTag.ExpirationTime:
+                case IptcTag.ReleaseTime:
+                    return true;
+
+                default:
+                    return false;
+            }
+        }
     }
 }

src/ImageSharp/Metadata/Profiles/IPTC/IptcValue.cs

@@ -103,6 +103,12 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Iptc
                     {
                         var cappedValue = value.Substring(0, maxLength);
                         valueBytes = this.encoding.GetBytes(cappedValue);
+
+                        // It is still possible that the bytes of the string exceed the limit.
+                        if (valueBytes.Length > maxLength)
+                        {
+                            throw new ArgumentException($"The iptc value exceeds the limit of {maxLength} bytes for the tag {this.Tag}");
+                        }
                     }
                     else
                     {

tests/ImageSharp.Tests/Metadata/Profiles/IPTC/IptcProfileTests.cs

@@ -16,7 +16,7 @@ namespace SixLabors.ImageSharp.Tests.Metadata.Profiles.IPTC
     {
         private static JpegDecoder JpegDecoder => new JpegDecoder() { IgnoreMetadata = false };
 
-        public static IEnumerable<object[]> allIptcTags()
+        public static IEnumerable<object[]> AllIptcTags()
         {
             foreach (object tag in Enum.GetValues(typeof(IptcTag)))
             {
@@ -25,7 +25,7 @@ namespace SixLabors.ImageSharp.Tests.Metadata.Profiles.IPTC
         }
 
         [Theory]
-        [MemberData("allIptcTags")]
+        [MemberData("AllIptcTags")]
         public void IptcProfile_SetValue_WithStrictOption_Works(IptcTag tag)
         {
             // arrange
@@ -41,6 +41,45 @@ namespace SixLabors.ImageSharp.Tests.Metadata.Profiles.IPTC
             Assert.Equal(expectedLength, actual.Value.Length);
         }
 
+        [Theory]
+        [InlineData(IptcTag.DigitalCreationDate)]
+        [InlineData(IptcTag.ExpirationDate)]
+        [InlineData(IptcTag.CreatedDate)]
+        [InlineData(IptcTag.ReferenceDate)]
+        [InlineData(IptcTag.ReleaseDate)]
+        public void IptcProfile_SetDateValue_Works(IptcTag tag)
+        {
+            // arrange
+            var profile = new IptcProfile();
+            var datetime = new DateTime(1994, 3, 17);
+
+            // act
+            profile.SetDateTimeValue(tag, datetime);
+
+            // assert
+            IptcValue actual = profile.GetValues(tag).First();
+            Assert.Equal("19940317", actual.Value);
+        }
+
+        [Theory]
+        [InlineData(IptcTag.CreatedTime)]
+        [InlineData(IptcTag.DigitalCreationTime)]
+        [InlineData(IptcTag.ExpirationTime)]
+        [InlineData(IptcTag.ReleaseTime)]
+        public void IptcProfile_SetTimeValue_Works(IptcTag tag)
+        {
+            // arrange
+            var profile = new IptcProfile();
+            DateTime datetime = new DateTimeOffset(new DateTime(1994, 3, 17, 14, 15, 16), new TimeSpan(1, 0, 0)).DateTime;
+
+            // act
+            profile.SetDateTimeValue(tag, datetime);
+
+            // assert
+            IptcValue actual = profile.GetValues(tag).First();
+            Assert.Equal("141516+0100", actual.Value);
+        }
+
         [Theory]
         [WithFile(TestImages.Jpeg.Baseline.Iptc, PixelTypes.Rgba32)]
         public void ReadIptcMetadata_Works<TPixel>(TestImageProvider<TPixel> provider)