In the sense of "every second employed productively" as a motto, so brevity is important. It's a little tongue-in-cheek. I don't mean something like "every second between now and then is important". Instead, I mean that every second of the day is important.
Tacitus tells us that at Nero's games, rowdy out-of-town yokels were disrupting events, holding everyone up and soldiers had to discipline them ...
ne quod temporis momentum ... praeteriret
so that no moment of time ... was lost
Perhaps you could make a motto out of this, something like:
nihil temporis momenti praeterit or nullum temporis momentum praeterit - no moment of time is lost/escapes/is forgotten
Also, Cicero writes in Pro Caecina (101):
non me praeterit
it does not escape my notice
And so maybe you could add in a me - nihil me temporis momenti praeterit - no moment of time escapes me
I also want to write something like "no moment neglected" but can't decide if that would be nullum momentum praeteriit or nullum momentum praeteritum [est] and so have to defer to those whose written skills are better than mine!
EDIT: I forgot that nihil takes a partitive genitive so I've amended my phrases accordingly and added an alternative using nullus
Perhaps the sentiment in Horace, Ode 1.1.20–21 could be adapted:
nec partem solido demere de die
'...and he doesn't decline to take away a portion of the business day.'
For example, you might say partem solido demere de die sperne, 'decline to take away a portion of the business day'; or, if you want to apply the idea to the full day, not just the business hours of it, omit solido, I suppose.
Another possibility just occurred to me, though I'm not sure it quite works: In letter 2.17, Pliny the Younger states that his Laurentine villa is close enough to Rome that he can put in a full, honest day's work, getting everything done that needs to be done, and still be able enjoy a nice evening's stay at the villa. The phrase he uses is:
salvo iam et composito die
'...without losing or leaving unfinished your day's work' [J. H. Westcott]
'...leaving the business of the day unimpaired and well ordered' [P. G. Walsh]
'...without having cut short or hurried the day's work' [Betty Radice]
I recommend taking a look at the question regarding "without further ado". To me it seems that the most appropriate phrase for your need given there would be dismissa mora, "delay having been abandoned".
There is also the phrase nulla mora interposita, "with no delay being placed" which might be useful. Both of these have can be read in two ways, as absolute ablatives (corresponding to the translations I gave) or as full sentences with an implicit est. The latter reading would give the translations "delay has been abandoned" and "no delay has been placed". This dual reading possibility is due to the feminine words of the first declension looking alike in nominative and ablative.
You may wonder why I chose to use mora ("delay") instead of a word for "second". For one thing, mora is short and idiomatic Latin. The more important reason, however, is that unit of time "second" is too new to be used in classical Latin, and the words secunda and secundum my dictionary suggests can too easily be mistaken for the number "second" (the one between the first and the third). Of course this source of confusion exists in English as well, but I feel that the risk is far bigger in Latin. I am not aware of a good word for a short moment that isn't too prone to misunderstandings. You could use hora, but it might be too easily interpreted as "hour", and that might be inappropriate for your purposes.
The two bolded phrases are my suggestions. If you want to go for ultimate brevity, I think dismissa mora is a good choice. (But, as always, there is a fair chance that someone else might have better suggestions.)
My two recommendations are more like "do not postpone anything" than "do not waste a second". I think the two convey the same idea as a motto. If you want more literal option, I would offer nulla secunda/hora amissa, "with no second/moment thrown away". This is a decent option, but secunda is easily read as a number (and hora as "hour") and this is not really classical Latin. Whether or not this is an issue is up to you.