申的是fall，接到summer Offer是怎么回事

申了SUNY SB 17fall CS的PhD，收到一个老师发来的邮件：
Hey, just wanted to say hi and tell you that I made you a summer offer
to join our lab. As you may know i am doing all kinds of things in
systems, cybersecurity and also applied theoretical foundations.

I am also the CEO of a startup now so very busy, but you seem smart and
I think you would have lots of fun at Stony Brook excluding summer beach
time :)

You don't have to feel obliged to work with me if you accept my offer,
in fact, i think it is important to talk to pretty much everybody before
you figure out what you want to do but it is nice to know that someone
makes you summer funding offers.

So there you go :)

have a fun day!


有几个疑问：
1. 这个应该不算正式通知吧，那么究竟是这个老师要给我Offer,还是说学校要给我Offer呢？
     收到邮件开始以为是老师个人决定，可是那又为什么说You don't have to feel obliged to work with me if you accept my offer呢？是说我可以跟别的老师吗？
2. 可能因为我确实是3月份毕业，所以他给了夏季的Offer,可是可不可以改成秋季的呢？
     因为之前拖延症犯了，在11月12月申请季的时候太忙，同时还要写毕业论文，就想着反正申的是秋季，不如晚一点写，延期3个月，从3月答辩改成6月答辩也不耽误，于是就不可能夏季去了啊。这个可以商量不？如果这个老师就是想要夏季的学生那么这个Offer是不是就没有了啊？