Styjun

I have a DateTime and need to get the days and hours in a decimal format. For example, for new DateTime(2009, 6, 19, 18, 0, 0); I need the days as 19.75. Day + (TimeOfDay.TotalHours / 24) seems to work but is there a straight or better conversion? Speed is important for this calculation considering I'm also using high-resolution dates.

Given...

DateTime time = new DateTime(2009, 6, 19, 18, 0, 0);

...what you have...

decimal totalDays1 = (decimal) (time.Day + time.TimeOfDay.TotalHours / 24);// 19.75M

...is already pretty terse, so I don't know how much you're wanting or expecting to improve that. You could use the TimeSpan.TotalDays property, but it takes a bit more work to set that up...

DateTime lastDayOfPreviousMonth = new DateTime(time.Year, time.Month, 1).AddDays(-1);
decimal totalDays2 = (decimal) (time - lastDayOfPreviousMonth).TotalDays;// 19.75M

I used BenchmarkDotNet to benchmark four different methods...

using System;
using BenchmarkDotNet.Attributes;

public static class Program

 static void Main(string[] args)
 
 BenchmarkDotNet.Running.BenchmarkRunner.Run<CalculateTotalDaysBenchmarks>();
 


[ClrJob()]
[CoreJob()]
public class CalculateTotalDaysBenchmarks

 private static readonly DateTime TestTime = new DateTime(2009, 6, 19, 18, 0, 0);

 [Benchmark(Baseline = true)]
 public decimal Method1_DayPlusTotalHoursDivided_CastResult()
 
 return (decimal) (TestTime.Day + TestTime.TimeOfDay.TotalHours / 24);
 

 [Benchmark()]
 public decimal Method1_DayPlusTotalHoursDivided_CastTotalHours()
 
 return TestTime.Day + (decimal) TestTime.TimeOfDay.TotalHours / 24;
 

 [Benchmark()]
 public decimal Method2_DayPlusTicksDivided()
 
 return TestTime.Day + (decimal) TestTime.TimeOfDay.Ticks / TimeSpan.TicksPerDay;
 

 [Benchmark()]
 public decimal Method3_SubtractLastDayOfPreviousMonth()
 
 DateTime lastDayOfPreviousMonth = new DateTime(TestTime.Year, TestTime.Month, 1).AddDays(-1);

 return (decimal) (TestTime - lastDayOfPreviousMonth).TotalDays;
 

 [Benchmark()]
 public decimal Method4_NewTimeSpan()
 
 return (decimal) new TimeSpan(TestTime.Day, TestTime.Hour, TestTime.Minute, TestTime.Second, TestTime.Millisecond).TotalDays;
 

...and got these results...

// * Summary *

BenchmarkDotNet=v0.11.4, OS=Windows 10.0.17763.379 (1809/October2018Update/Redstone5)
Intel Core i7 CPU 860 2.80GHz (Nehalem), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=2.1.505
 [Host] : .NET Core 2.1.9 (CoreCLR 4.6.27414.06, CoreFX 4.6.27415.01), 64bit RyuJIT
 Clr : .NET Framework 4.7.2 (CLR 4.0.30319.42000), 64bit RyuJIT-v4.7.3362.0
 Core : .NET Core 2.1.9 (CoreCLR 4.6.27414.06, CoreFX 4.6.27415.01), 64bit RyuJIT


| Method | Runtime | Mean | Error | StdDev | Ratio | RatioSD |
|------------------------------------------------ |-------- |---------:|----------:|----------:|------:|--------:|
| Method1_DayPlusTotalHoursDivided_CastResult | Clr | 118.2 ns | 1.2644 ns | 1.1827 ns | 1.00 | 0.00 |
| Method1_DayPlusTotalHoursDivided_CastTotalHours | Clr | 263.9 ns | 0.7289 ns | 0.6462 ns | 2.23 | 0.02 |
| Method2_DayPlusTicksDivided | Clr | 194.1 ns | 0.8827 ns | 0.8256 ns | 1.64 | 0.02 |
| Method3_SubtractLastDayOfPreviousMonth | Clr | 138.9 ns | 0.4757 ns | 0.3714 ns | 1.17 | 0.01 |
| Method4_NewTimeSpan | Clr | 134.7 ns | 0.8376 ns | 0.7835 ns | 1.14 | 0.01 |
| | | | | | | |
| Method1_DayPlusTotalHoursDivided_CastResult | Core | 113.3 ns | 0.1982 ns | 0.1655 ns | 1.00 | 0.00 |
| Method1_DayPlusTotalHoursDivided_CastTotalHours | Core | 261.3 ns | 2.9683 ns | 2.6313 ns | 2.31 | 0.02 |
| Method2_DayPlusTicksDivided | Core | 197.9 ns | 4.4254 ns | 5.2681 ns | 1.74 | 0.04 |
| Method3_SubtractLastDayOfPreviousMonth | Core | 131.1 ns | 0.8406 ns | 0.7863 ns | 1.16 | 0.01 |
| Method4_NewTimeSpan | Core | 132.1 ns | 1.1211 ns | 1.0486 ns | 1.16 | 0.01 |

What you started with is appreciably faster than the other methods.