It may not be safe to cast valueData to String and/or to call trim -- it may be null, especially if nodes[i]'s optype is not ExpressionValue. Test first.

You might want to add some junit test cases:
Add one that uses "||" to build up a string argument to since_epoch (that is not a constant value) -- that case should "continue;" here and (if executed) get an error in the EE, especially if it builds a bad date/time string.  
Add one that passes a value of the wrong type --  like a constant decimal number -- to since_epoch -- I THINK that should get an incompatible data type error here.

longtime is a misleading name -- the string does not contain a long value - call it datetimestring?