This formula can help:
=COUNTIFS(K:K,"abc",J:J,">="&J2,J:J,"<="&J15)
Where K:K is the range where you have the usernames and "abc" is a sample name. And J:J is the timestamps range. J2 contains begin date and J15 contains end date for the required range.
Make sure date in J2 is lower than the date in J15 otherwise it will give 0 as answer