Tracking your time rather than reconstructing it at the end of the month turns out to be really important. As in, not-overbilling-your-clients-by-23% important. Whether or not you share your time records with your clients in real time, you do need to keep a time log.
Hopefully that is a no-brainer for everyone who reads this post, but I doubt it. I have known plenty of lawyers who reconstruct time at the end of every month — or every couple of months — from their calendar, emails, and memory. Even if you only put together your bill at the end of the month, you need to track your time as you go. These are the major tools for doing that.
However you decide to track your time, pick a method and stick to it. When you sit down to assemble your invoices, the fewer places you have to go to get the raw data, the better. The more methods you use to track your time, the more mistakes you will make.