Add Time from TimeSpan Object in Powershell - powershell

Question

This has been driving me nuts, and it's got to be something simple I'm missing.
I'm simply trying to get the total hours from a TimeSpan object and use that to add to a DateTime object.
$mytime = "1/1/2020 10:00:00"
$hours = (New-TimeSpan -Start $mytime).TotalHours
Get-Date
(Get-Date $mytime).AddHours($hours)
The result is
Wednesday, February 5, 2020 11:25:37 AM
Wednesday, February 5, 2020 11:25:37 AM
In the above, the time doesn't change! However if I just assign a value to $hours, it works fine.
$mytime = "1/1/2020 10:00:00"
$hours = 100
Get-Date
(Get-Date $mytime).AddHours($hours)
Wednesday, February 5, 2020 11:25:37 AM
Sunday, January 5, 2020 5:00:01 AM
I've tried all manner of variable assignments and casting, but no matter what I do I can't get the TotalHours value to work. Why can't I use $hours? TotalHours is supposed to be a normal double, and even if I cast it to double ([double]$hours = New-Timespan...) nothing changes.

Answer 1

TotalHours includes fractional hours. The value of $hours will be something like 844.1311..... The value of TotalHours is basically:
$ts = New-TimeSpan -Start $mytime
$ts.Ticks / [TimeSpan]::TicksPerHour
If you want the integer value, try:
$mytime = "1/1/2020 10:00:00"
$hours = [math]::Truncate((New-TimeSpan -Start $mytime).TotalHours)
(Get-Date $mytime).AddHours($hours)
I haven't tested it, but you might need to use Math.Floor() instead if you can potentially have $mytime in the future.
Truthfully, though, if you want the same hour of day today as another date, it's more logical to do this:
$mytime = Get-Date "1/1/2020 10:00:00"
$today = (Get-Date).Date
$today.AddHours($mytime.Hour)
Or:
$mytime = Get-Date "1/1/2020 10:00:00"
[DateTime]::Today.AddHours($mytime.Hour)

Answer 2

It works for me. You're adding 844.4 hours (35 days) in the first example, the number of hours to get to the current day and time, and 100 hours (4 days) in the second.
$mytime = "1/1/2020 10:00:00"
$hours = (New-TimeSpan -Start $mytime).TotalHours
$hours
844.404845696333
(Get-Date $mytime).AddHours($hours)
Wednesday, February 5, 2020 2:24:17 PM
$mytime = "1/1/2020 10:00:00"
$hours = 100
(Get-Date $mytime).AddHours($hours)
Sunday, January 5, 2020 2:00:00 PM
Here's another example:
[datetime]'2:28pm' + (new-timespan -Hours 1)
Wednesday, February 5, 2020 3:28:00 PM
If it's under 24 hours, it can be as simple as ([timespan] is implied in the 2nd argument):
[datetime]'2:28pm' + '1:0'
Wednesday, February 5, 2020 3:28:00 PM
Subtracting two datetimes:
[datetime]"Wednesday, February 5, 2020 11:25:37 AM" - [datetime]"1/1/2020 10:00:00"
Days : 35
Hours : 1
Minutes : 25
Seconds : 37
Milliseconds : 0
Ticks : 30291370000000
TotalDays : 35.0594560185185
TotalHours : 841.426944444444
TotalMinutes : 50485.6166666667
TotalSeconds : 3029137
TotalMilliseconds : 3029137000