An indication is the correct label if you hover your mouse over the field in table view (CTRL-T). It should say timestamp, numerical (etc).
Then, by default the field values will be differently aligned (right) than pure text values (left).
If you use num(FIELD), you get the numerical representation returned, a number. If you use text(FIELD), you get the textual representation.
A QV timestamp is a dual and should have both representations (the numerical representation is the reason you can use numerical operators like +,- on timestamps or dates).
If your field is not correctly interpreted, use a correct standard format setting in your script
SET TimestampFormat='M/D/YYYY h:mm:ss[.fff] TT';
or use the QV functions timestamp#() resp. date#() to interpret your input values. Please check these functions and the date and time format codes.
There was also recently a nice blog entry by Henric Cronstroem explaining all this in detail.