How do I set a specific start time and date for an imported text file?

You can specify the time at which the first sample in the file occurs by manually entering a time value within the .txt file. The time needs to be in a form that LabChart understands, and the steps to do this are as follows:
1. Calculate the desired time in Excel format (this is represented as the number of days from the first day of the year 1900)! An easy way to do this is to use the DATE(year, month,day) and TIME(hour, minute,seconds) functions in Excel. For example, the cell formula: =DATE(2010,8,4)+TIME(10,30,4.7) returns 40394.43750 which is the time: 10:30 am, 4th August 2008. Use the 24-hour clock format for times in the pm.  Set the cell's format to 'Number' and increase the displayed decimal places appropriately. 
2. Open the .txt file you're importing in a text editor. At the top of the file, add a row such as: ExcelDateTime= {press tab key} 40394.4375000 
In this example, the tab key is used to create a space between ExcelDateTime= and the desired file start time .  An additional line can be added that sets the time axis in LabChart to have minor divisions of .001 seconds: Interval= 0.001 s.  The use of this additional line is optional, but allows for a cleaner looking time scale.