What is the correct test? dates included


I have a table with dates (every day, ascending order), where each row is the flowing day, and a column with number of expressions. Something like this:
Day 1 | 20 expressions
Day 2 | 23 expressions

I need to see if there is significance between two periods: up to day 595, and from day 595 onward. What is the best way to do it?
I thought assigning a new variable and setting its value to 0 for each day in the first period and 1 for each day in the second period, and then do a t-test over the expressions. Would this be ok?

Many thanks for your help!