How do I add leadding zeros for my timespan date parts when I need them?

I have a TimeSpan object that I'm breaking into its individual parts  (days, hours, minutes, seconds), as strings, using the ToString() method. How do I ensure that for each part that is a single digit, that I add a leading 0? for example, this: 1:6:45:9 would become this: 01:06:45:09

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TimeSpan ts = currentDateTime - signInTime;


return ts.Days.ToString() + ":" + ts.Hours.ToString() + ":" + ts.Minutes.ToString() + ":" + ts.Seconds.ToString();
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