We can use the Present Perfect with time expressions which mean up to and including now.
eg. over the past few days/months/years
during the last couple of days/weeks/months
up until now
We can use the Past Simple with definite time expressions in the past.
eg. yesterday, a few weeks ago, last year, in 1970, etc
Some time expressions can be used with both the Present Perfect and the Past Simple.
A: I answered about 20 emails at work today.
B: I’ve answered about 20 emails at work today.
A: I saw a lot of movies during the school holidays.
B: I’ve seen a lot of movies during the last few days.
A: Since I finished university I’ve been to a lot of job interviews.
B: I’ve been to a lot of job interviews since I’ve been unemployed.
A: As soon as I finished work, I went to the gym.
B: As soon as I’ve finished work, I’m going to the gym.
In all the above cases, speaker A uses the Past Simple because the event was completed at a definite time in the past or he/she considers the period of time as finished.
Speaker B uses the Present Perfect because the time period or event is still continuing or he/she considers the period of time as still continuing.
NB: As soon as can be replaced with once/when/after. eg. Once I’ve finished work, I’m going to the gym.
During can be replaced with in. eg. I saw a lot of movies in the school holidays.
We use up until/ until/ till/ up to + now eg. I’ve directed three movies up until now.
We use It’s the first time with the Present Perfect to say when something happens for the first time. eg. It’s the first time I’ve been on a plane.
We can say This is/ that’s the first/second/third,etc time with the Present Perfect. This is the second time I’ve been to Istanbul.