~~~
item replace entity @s inventory.18 with minecraft:tipped_arrow[minecraft:custom_name='{"translate":"item.minecraft.tipped_arrow.effect.slowness","color":"white","italic":false}', minecraft:potion_contents={custom_effects:[

]}] 32
~~~
The above command help you to get a tipped arrow with slowness IV effect for 80tick. However, the display of the item will say 00:00(should be 00:04). When you adjust the duration to 2000, it will say 00:12(should be 01:40). 
 
But I have tested that the real time works and it's truely give you 4s slowness IV when you're shot. Only the display is wrong.
 
This is a bug which can't reveal the correct duration of custom tipped arrow.