Answer on your question depends on the type of the deadline. Substantive deadline can be determined by the law or can be stipulated in the contract. But it all depends on the legal domain. For example, in the public law domain general rule is that you are not allowed to extend the deadline. You need to comply with all the requests and send all the relevant documents within particular timeframe.
Procedural deadlines usually are not extendable. There are few exceptions but in the general context are not relevant. Every procedural deadline must be met.
From the hastags that you have mention in the post, I assume that you must met procedural deadline. In order to do so, be aware that you can deliver documents to the public body personally or you can use service of the authorized postal service provider. Be aware that Croatian post is the only authorized postal service provider. So if you hand documents to the Croatian post service to deliver it to the public body, that action will be treated just like you handed personally the documents to the public body even if you hand them documents on the last day of the deadline.
Same rules with the authorized postal service provider apply for the substantive deadlines.
Every deadline expires in exactly 00:00 a.m. on the following day.
If the last day of the deadline is on holiday or non working day, by the law, the deadline is extended to the first work day.
Hope this can help.