How to address mutable parameters like datatime in Powershell?

We have a server that is running out of space (435Gb remaining) and we want to know when it will as new 1Gb files are created every 15 hours consistently. So, I've crafted a script where users can enter a start date, and units of 15 hours. The script will then display the date and time after the two parameters are provided. Challenge I am having is that Powershell returns and out of range error. Is AddHours somehow violating the mutability of the date provided? Not sure how I can get around that.

# enter start date$startDate = Read-Host "Enter the start date (MM/dd/yyyy)"# enter in increments of 15 hours$increments = Read-Host "Enter the number of 15-hour increments"# convert the start date$startDateObj = [DateTime]::ParseExact($startDate, "MM/dd/yyyy", $null)# calculate future date applying 15 hr increments$endDateObj = $startDateObj.AddHours($increments * 15)# display the end date and timeWrite-Host "End Date and Time: $($endDateObj.ToString("MM/dd/yyyy hh:mm tt"))"

This is the error generated

Exception calling "AddHours" with "1" argument(s): "Value to add was out of range.Parameter name: value"At line:11 char:1+ $endDateObj = $startDateObj.AddHours($increments * 15)+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : NotSpecified: (:) [], MethodInvocationException+ FullyQualifiedErrorId : ArgumentOutOfRangeExceptionYou cannot call a method on a null-valued expression.At line:14 char:34+ ... t "End Date and Time: $($endDateObj.ToString("MM/dd/yyyy hh:mm tt"))"+                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : InvalidOperation: (:) [], RuntimeException+ FullyQualifiedErrorId : InvokeMethodOnNull