Set your math column like this.
Now + 360/1440
or
Now + 6/24
or
Now + 0.25
Date math in Glide is based around days, so if you want to add a certain number of hours or minutes, you need add the fractional number of minutes out of the total number of minutes in a day.
For example, there are 1440 minutes in a day, or 24 hours on a day.