An alternative that literally converts the "H" to ":" = --SUBSTITUTE( LEFT(time.text, LEN(time.text)-1), "H", ":")
In addition the '--' coerces the text rendering to a number. That makes it easier to calculate quantities such as the duration in days or the end date. = INT(Duration)
BTW, 'time.text' is just the name I have given to the time column. For me that creates a dynamic array formula but older versions of Excel interpret it differently and use implicit intersection to reduce the array calculation to a sequence of single-cell calculations.