Format DateTime strings as correct ISO8601 (#296)

Fixes #295 

Turns out that the default string formatting of DateTime objects doesn't
conform to ISO8601, so even when the object is saved as UTC; the app
does not recognize it as UTC.

coffeecard/CoffeeCard.Library/Services/v2/AccountService.cs

@@ -184,7 +184,7 @@ private async Task AnonymizeUserAsync(User user)
             user.Name = string.Empty;
             user.Password = string.Empty;
             user.Salt = string.Empty;
-            user.DateUpdated = DateTime.Now;
+            user.DateUpdated = DateTime.UtcNow;
             user.PrivacyActivated = true;
             user.UserState = UserState.Deleted;
             await _context.SaveChangesAsync();

coffeecard/CoffeeCard.Library/Services/v2/VoucherService.cs

@@ -42,7 +42,7 @@ public async Task<IEnumerable<IssueVoucherResponse>> CreateVouchers(IssueVoucher
                 .Select(code => new Voucher
                 {
                     Code = code,
-                    DateCreated = DateTime.Now,
+                    DateCreated = DateTime.UtcNow,
                     Product = product,
                     Description = request.Description,
                     Requester = request.Requester

coffeecard/CoffeeCard.Library/Utils/DateTimeConverter.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace CoffeeCard.Library.Utils
+{
+    public class DateTimeConverter : JsonConverter<DateTime>
+    {
+        private const string DateFormatIso8601 = "yyyy-MM-ddTHH:mm:ss.fffZ";
+
+        public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            return DateTime.Parse(reader.GetString()!, CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal);
+        }
+
+        public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializerOptions options)
+        {
+            writer.WriteStringValue(value.ToUniversalTime().ToString(DateFormatIso8601, CultureInfo.InvariantCulture));
+        }
+    }
+}

coffeecard/CoffeeCard.Tests.Unit/Utils/DateTimeConverterTest.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Text.Json;
+using CoffeeCard.Library.Utils;
+using Xunit;
+
+namespace CoffeeCard.Tests.Unit.Utils
+{
+    public class DateTimeConverterTests
+    {
+        private readonly JsonSerializerOptions _options;
+
+        public DateTimeConverterTests()
+        {
+            _options = new JsonSerializerOptions();
+            _options.Converters.Add(new DateTimeConverter());
+        }
+
+        [Fact]
+        public void Serialize_ShouldFormatDateTimeCorrectly()
+        {
+            // Arrange
+            var dateTime = new DateTime(2022, 1, 9, 21, 3, 52, millisecond: 123, DateTimeKind.Utc);
+            var expectedJson = "\"2022-01-09T21:03:52.123Z\"";
+
+            // Act
+            var json = JsonSerializer.Serialize(dateTime, _options);
+
+            // Assert
+            Assert.Equal(expectedJson, json);
+        }
+
+        [Fact]
+        public void Deserialize_ShouldParseDateTimeCorrectly()
+        {
+            // Arrange
+            var json = "\"2022-01-09T21:03:52Z\"";
+            var expectedDateTime = new DateTime(2022, 1, 9, 21, 3, 52, DateTimeKind.Utc);
+
+            // Act
+            var dateTime = JsonSerializer.Deserialize<DateTime>(json, _options);
+
+            // Assert
+            Assert.Equal(expectedDateTime, dateTime);
+        }
+
+        [Fact]
+        public void Serialize_Deserialize_RoundTrip_ShouldMatchOriginalDateTime()
+        {
+            // Arrange
+            var originalDateTime = new DateTime(2022, 1, 9, 21, 3, 52, DateTimeKind.Utc);
+
+            // Act
+            var json = JsonSerializer.Serialize(originalDateTime, _options);
+            var deserializedDateTime = JsonSerializer.Deserialize<DateTime>(json, _options);
+
+            // Assert
+            Assert.Equal(originalDateTime, deserializedDateTime);
+        }
+    }
+}

coffeecard/CoffeeCard.WebApi/Startup.cs

@@ -122,6 +122,7 @@ public void ConfigureServices(IServiceCollection services)
                     options.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
                     options.JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.Never;
                     options.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter());
+                    options.JsonSerializerOptions.Converters.Add(new DateTimeConverter());
                 });
 
             services.AddCors(options => options.AddDefaultPolicy(builder =>