it says "visa expire on 30 April'. Maybe a stupid question but is 30 April included in my current visa?

http://www.immigration.go.th/nov2004/content/visaxpiredate.html says:

What does the Visa Expiration Date Mean? The visa expiration date is shown on the visa. Depending on the alien’s nationality, visas can be issued for any number of entries, from as little as one entry to as many as multiple (unlimited) entries, for the same purpose of travel.

• This generally means the visa is valid, or can be used from the date it is issued until the date it expires, for travel for the same purpose, when the visa is issued for multiple entry.
• This time period from the visa issuance date to visa expiration date as shown on the visa, is called visa validity. If you travel frequently as a tourist for example, with a multiple entry visa, you do not have to apply for a new visa each time you want to travel to Thailand.
• The visa validity is the length of time you are permitted to travel to a port-of-entry in Thailand to request permission of the immigration officer to permit you to enter Thailand. The visa does not guarantee entry to Thailand.
• The Expiration Date for the visa should not be confused with the authorized length of your stay in Thailand, given to you by the immigration officer at port-of-entry. The visa expiration date has nothing to do with the authorized length of your stay in Thailand for any given visit.
• Each time you arrive at the port-of-entry, an immigration officer decides whether to allow you to enter and how long you can stay. Only the immigration officer has the authority to permit you to enter Thailand.

It is not the visa itself you need to worry about, but the 'permission to stay' on the stamp you received on arrival.

You must leave on or before that date to avoid overstay.

EDIT If you have a visa with more than one entry, all entries must be made before the 'enter before' date on the visa (effectively the visa expiry date).

Thanks for you answer. Currently got an almost finished non immigrant B visa mentioning those words.

If you have a visa it has a enter before date on it that is date the date your visa expires.

You will also have a entry/permitted to stay stamp in your passport that the immigration officer did on entry. That has the important date for being on overstay or not on it.

it says "visa expire on 30 April

You have to enter the country using the visa by 30 April.

Once you're in the country, that date is not important. When you entered the country, the immmigrations officer stamped your passport giving you permission to stay in the country. Look at the date on the hand stamp he put in your passport to see when you need to leave the kingdom.