Voting Question: What are some tips in troubleshooting a "select case" problem in VBA for excel?

I have a select case that uses a variable "starttime" to be put in the appropriate section of my spreadsheet. It works well for some, but for others it just skips the appropriate case, even though I have a msgbox beforehand telling me that the correct value of starttime is obtained, and as I use the "step into" feature I can watch it just go past the correct case. Example, starttime is 6:00:00 AM and it gets correctly placed, the loop adds 30 min and then runs through the select case again. 6:30:00 AM is also correctly placed. Third time, 7:00:00 AM goes though and does not get correctly read into the appropriate case. I have triple checked for typos, and believe that it might be due to round off error of some sort. HELP!