The date field is probably not one day behind it stores the value in UTC therefore if in the UK it would store 03/10/2017 00:00 as 02/10/2017 23:00 as we are currently UTC +1 you can convert to local time using ToLocal.
More information on it here but you can see it clearly in the database.
ToLocal function: HEAT Service Management Online Help
and a bit on Date/Time Zones: https://help.ivanti.com/ht/help/en_US/ISM/2017/Index.html#ServiceDesk/Dates/Working with Dates Times and Time Zones.htm
Thank you, this is exactly what I needed!