Static checker invariant unproven on timespan field

Question

I am attempting to implement code contracts for a class with TimeSpan properties where I want to ensure that the TimeSpan is non-negative.  With the following code the static checker is giving a warning that the contract invariant is unproven (the ensures
and requires seem to be fine).  A suppress message is suggested as follows:
CodeContracts: WiseTechGlobal.ROPE.Common: Method WiseTechGlobal.ROPE.Common.DepotHours(): To mask the warning at WiseTechGlobal.ROPE.Common.DepotHours.#ctor()[0x11] add
the attribute: [SuppressMessage("Microsoft.Contracts", "Invariant-16-17")]
I'd appreciate any help or guidance on how to correct this issue.  I'd rather not suppress the message if there is a change I can make to correct my contract invariant test. One thing I have tried
is to initialize the backing field with the following, but this did not help.
TimeSpan earliestTimeVehiclesCanStart = new TimeSpan(0);
using System;
using System.Diagnostics.Contracts;
namespace WiseTechGlobal.ROPE.Common
// Defines the operating hours and loading/unloading times for a depot
public class DepotHours
public TimeSpan EarliestTimeVehiclesCanStart
get
Contract.Ensures(Contract.Result<TimeSpan>() >= TimeSpan.Zero);
return earliestTimeVehiclesCanStart;
set
Contract.Requires<ArgumentException>(value >= TimeSpan.Zero);
earliestTimeVehiclesCanStart = value;
TimeSpan earliestTimeVehiclesCanStart = TimeSpan.Zero;
[ContractInvariantMethod]
void ObjectInvariant()
Contract.Invariant(earliestTimeVehiclesCanStart >= TimeSpan.Zero);

Answer 1

The checker doesn't understand that TimeSpans work much like integers, so it has no clue that the initialization "x = TimeSpan.Zero" guarantees "x >= TimeSpan.Zero" or even that "x == TimeSpan.Zero". One workaround for this
is to simply tell the checker that this is so:
TimeSpan earliestTimeVehiclesCanStart;
DepotHours() {
earliestTimeVehiclesCanStart = TimeSpan.Zero;
Contract.Assume(earliestTimeVehiclesCanStart >= TimeSpan.Zero);
Note that it's not sufficient to tell it that "earliestTimeVehiclesCanStart == TimeSpan.Zero", because it can't prove that "==" implies ">=" for TimeStamp instances either.