Three dates appear on an e-visa, and they do not mean the same thing. Understanding the difference is what keeps you on the right side of an overstay fine.
Issue date
The day the visa was granted. It has no bearing on when you may enter.
Valid from
The arrival date you nominated on the form. You cannot enter before it. If you arrive a week early because a flight changed, the visa will not be accepted.
Valid until
The last day you may remain in the country. For a 30-day visa this is 30 days from the valid-from date — not from the day you actually arrive. Entering three days late costs you three days of stay.
The stamp overrides your assumptions
Whatever you calculated at home, the date written into the entry stamp is what immigration enforces. Read it before you leave the counter.