There was a discussion today at a conference I was at about the difficulty of keep track of your time when you bill by the time increment. I strongly recommend tracking as you go and not waiting until the end of the day or the next day for the sake of accuracy and to avoid the headache of trying to reconstruct what you worked on previously.
In order to assist with that, I recommend keeping your list of billing codes on your computer desktop or on your actual desktop so you have easy access to it. If you don't have billing codes, I recommend coming up with a consistent short-form of your accounts, like the first three letters of the person or entity or project you are working on and the year they become a client, for example (like McG18).
You can keep track easily from there with a simple Excel spreadsheet. If you simply use the columns of "date, billing code, time start, time stop, time spent", you can fill in the first 4 as you go along. As you switch projects, before you start your new one, enter your time stop and enter the new billing code you are about to work on. It honestly becomes a habit, but it takes a few difficult days getting used to doing it.
However, you don't have to go the "home grown tracking system route," though. There are some really great tracking programs and apps available out there for you, as well:
1. QuickBooks has a system called TSheets. It does charge per user ($8/person plus a base fee of $20, as of this writing). You are paying for the reliability of QuickBooks and the integration with their system, as well as the convenience -- the app version is available on all mobile devices.
2. BigTime is another one that is available online and mobile (iOS and Android). This one can integrate with QuickBooks as well, but can also integrate with Slack and others. It does have a variety of pieces, but specifically of use is a time tracking system that can generate reports.
3. TimeSolve Legal Software is specifically designed to help you track time by the minute, like an attorney. It is cloud-based and comes as a monthly subscription that has support via email and phone.
4. Mango Billing is an interesting program that doesn't just track time, but it can save client contact information, track email logs, and schedule meetings.
There are honestly tons more from there, too, but I wanted to give you a starting point of some options. You can go home-grown, or you can go with one of the many systems available out there, but make sure that however you do it, if you need to track your time, you make it an automatic part of your day and as easy as it can be for you!