Way To Make Clause On Sargable?

I have the following query

update r
set Comp = t.Comp
from [QlikDataWarehouse].[dbo].[Vente]r
inner join
[QlikDataWarehouse].[dbo].[Budget] t
on t.[Code Site] = r.[Code Site] and
t.[Code Rayon] = substring(r.[Code Structure],1,4) and
t.[Date Time] = convert(date,r.[Date Time])
where r.[Date Time] >= '2015-01-01 00:0:00.000';

number of rows of budget is around 800000 and vente around 350+millions rows . I create index vente([Date

Time], [Code Site], [Code Structure]) and Budget([Code
Site], [Code Rayon], Date Time]).

But the query took days running .

t.[Code
Rayon] is an int type t.[Date
Time] is a date type r.[Date
Time] is datetime type

Is there a way to make clause On sargable ?