How to Join 2 tables if datetime diff is within range?

Hi, problem figuring out how to join tables if the datediff ? is within range.  

There are 3 tables  a main table then 2 sub tables, these are measurements with a date/time value.  Im trying to take the datetime entries from the 2 sub tables and match them, then add  them to the main table

subtable1 has SampleID, datetime, 

subtable2 has TesterID, datetime

maintable has SampleID, datetime, TesterID, datetime

so the maintable would always have the sampleID value and datetime regardless.  but it would only JOIN the testerID and its datetime, only IF these 2 datetime values were close, and the datediff interval was set to 1 second.

 its a function call that fills the main tables matching columns but only if the testerIDs datetime is close to the sampleIDs time????

thanks