Hi there, I hope this explanation helps! Both of the sentences are possible, but it would depend on wider context which is best. If you say any adverb of time which refers to the present, e.g. today, this year, this week, this month, you have a choice. It all depends on whether you think that there is a chance that the event will happen again or not within the time frame. For example: I saw her today can mean that you think there is no chance that you will see her again today. - maybe it is the end of the day so you won't see her again - maybe you know her from work, and you have already left the office so you know you won't see her again. I have seen her today can mean that you think that you may see her again. - maybe you live with her and someone is asking you where she is. If it is midday you can say that you have seen her today, so she is definitely somewhere in the area and therefore either you or the person you are speaking to should see her later. Here are some more examples: - I have had 2 great holidays this year (there is still a few months left and the possibility to go on more holidays) - I had 2 great holidays this year (it is December, the year is nearly over, there is no chance that you will go on holiday again) - The photocopier has broken down twice this week (It is only Wednesday, so many chances for it to break down again) - The photocopier broke down twice this week (you are telling this you your friends on a Friday night, the week is over so it won't break down again)