I have a small project in which I would like to create multiple countdown timers in excel however I'm fairly new to excel macros. Would love some assistance.

Basically what I'm trying to get is cells a1:a100 are usernames and b1:b100 are countdown timers. All timers would start at the same number... ex 10 mins. Could anyone help me create a macro in which I could chose a1 and that starts the countdown in b1, a2 starts b2, and so on. This would be going on at different times.This part I want to stay manual. Ie b1 starts at 8:45,b2 starts at 8:46. I would like to choose the start time of the countdown.
Point me in the right direction, send me a tutorial link, do it for me and let me learn on my own, teach me. Anything will help.

Thanks for your help in advance.
-Dawid