A routine cannot do it, but if you use Smart Lighting you can. (This is an official feature.)
Open the mobile app.
Go to the marketplace (all the apps there are free at this time), which is the *Icon in the lower right.
Choose smart apps
Choose either smart things recommended or lighting
Choose smart lighting and create a rule (which smartthings calls an automation)
Select the light you want to have come on
Say that you want to trigger it with a specific time
You will then also be given an option to have it turn off, in your example 45 minutes after it comes on.
I can’t give you the screenshot because since the last iOS mobile app update for SmartThings it is no longer navigable by voice over, but maybe someone else can.
So you can do it with one smart lighting automation as long as it is based on a specific time. But you still can’t do it based on sunset except with the suggestion that @dalec made previously.
The following might be of interest: