RadMaskedDateTimeInput uses internally DateTime? (nullable DateTime) property to store its Value.
In other words, the Value property of this control is from type DateTime? and it cannot be directly bound to TimeSpan property. At least you need a converter. Since your binding does not succeed (due to mismatch of types), the fallback value which is DateTime.Now is being used.
So our advice is to bind the Value of the DateTimeInput to property of type DateTime?. Let us know if this fits well in your scenario.