Should Buddhists give their time to helping others?

First I think that it is a good idea to examine title of this post, and point out that there is no real "should" that we encounter in Buddhism, there are encouragements, but no commandments; or more politely, because of the truth of emptiness, or lack of inherent meaning and essence, whenever we say "should," in order for it to be a valid statement, we have to add a contextual qualifier. So, "Should we do X?" always becomes, "If we want X, should we do Y?" or something to that effect. Given that, your question lacks a qualifier, you asked, "Should we do X?" without offering a "Y". So, why?

Now, about time. I would offer that, in my understanding, time is, indeed a conditioned phenomena, it is not absolute (there is no absolute time, "it" flows at different rates in the universe), it relys on other phenomena for its existence (such as space, therefore objects, gravity, ect.) Because of this, time is compounded. However, you cannot give or take it in any real sense. It is intangible, unlike other compounded phenomena, a chair for instance, it is purely abstract with no appearance to lock it into our experience with, it is not a real thing in the same way that a chair is. The best we can do is try to record and measure the passing of events with a chronometer, but there is no thing called "time" that we can actually measure, we just call the passing of events that, nominally inferring time into existence, in retrospect, or in anticipation.

The metaphor of "giving time" is merely a convention that we use for service, in the certain light of sacrificing our own agenda. I would, again rephrase the question in order to reflect the implied sacrifice of personal agenda.

So taking both points, the lack of qualifier in the first, and the inferential nature of the statement that calls for clarity in the second, I would suggest restating the question like this, "If we want to hasten our own liberation, should we practice service, sacrificing our own agenda of self centered goal seeking in order to be of benefit to others?"

So... When we put ourselves into service, when we put others, and the agenda of others, before ourselves and our own agenda, we undermine the conceptual self centered impulse. We undermine our tendency towards an ego-centric agenda. This is very helpful to our liberation, and it makes the world a better, happier place, so why not? It can eliminate negative karma, crate good karma, help see through the Samsaric delusion, and so forth. In this sense, it seems that being of service is a good personal agenda, as it has the potential to liberate us from our suffering.

Interestingly, this seems to leave us in a bind, how can I be of service to you, getting past my own self serving agenda, when that creates a self serving agenda as soon as I realize that I am doing it to get past my own suffering?

Well, this is where intention comes in. How compassionate are we being? The Dalai Lama talks about wise vs. foolish selfishness. Wise selfishness is realizing this, and being of service, giving back to the community because you know it will benefit you, and this is better than the alternative... ignoring this and being a glutton. Regardless, it is simple enough to say that the more your heart is in it, the more benefit it will be to you, and the more you practice it, the more your heart will be in it.

When we realize that helping people really means empowering them to overcome their own suffering, we become little "helper-makers", also known as teachers. In this way, we can realize selflessness giving rise to selflessness, and so forth.

TL;DR It is highly encouraged that we learn to put others before ourselves, and learn to be of true service.

The Four Dharma Seals encompass the whole Buddhadharma and are excellent. It brings me joy that you are learning them.

You should pretty much give everything, to perfect generosity. However, before you go out and give away your house and all your clothes, you can probably benefit people in greater ways by "setting your intention and being natural" -- one of the Lojong Mind Training verses. (You can read more about Lojong proverbs at this wonderful resource http://lojongmindtraining.com/)

Think about, in the simple example, of a man who could use his great wealth to buy many acres of farm land and supply his village with a seemingly endless supply of food. Had he given all his wealth away at once, it might not be as beneficial to sentient beings.

So, yes, time is definitely something you should give to other beings. In fact, practicing, studying, reflecting, and meditating can be of the most generous uses of our time, by setting our intentions to ultimately benefit all beings (as numberless as the sky is vast).

This setting of intention is called Bodhichitta, it is the "Mind inclined to Awakening" and it is so very very precious. More valuable than all the diamonds and gold and silver.

Your question is really wonderful! Our time is valuable, and if we can spend even 10 minutes a day thinking about how we can benefit someone we know, it is time well spent!

Also wonderful, is that using your time in the service of others lessens our attachment, and brings us closer to the natural.

You can use your time to think about how you can help people you know (or just met) who have similar needs as you. If you know someone without a job, you can try and help them get one, or if you know someone lonely, you can be their friend and really listen and be there for them.

You can also spend your time reflecting happily on the good deeds you've done for others. This is also a great way to uplift the mind and ensure that our practice of virtue grows and grows.

You can plant great seeds by encouraging the people you know to do the same: to help where help is needed, to think about how we can help others, and to spend our time offering all the wisdom and love we can.

To evaluate this question correctly, it is essential to look at the difference of Theravada and Mahayana, since answers given by people from these differing traditions are bound to be slightly different also. It appears to me, the four seals are a mahayanic equivalent to the (not-only) theravadic three marks of existence. Also the Theravada/Mahayana-difference is also highly relevant to the question of time being compounded.

Essentially this difference consists the ideal that people strive after, which is the Arahant in case of Theravada - someone who destroys all bondages and gains enlightenment for himself and the Bodhisattva in Mahayana - someone who strives to enlighten all beings before attaining Nirvana himself or herself. (I'm being very brief here.)

The interpretation you refer to, charity as the answer to impermanence of things sounds to me to be typically mahayanic or vajrayanic (Vajrayana developed out of Mahayana).

About time: this is one of the trickiest subjects of Buddhist thought. Thinking over the Buddha's denial of ultimate ontological status for compounded entities (like "the person"), Buddhist thinkers tried to find out, what in the end has this ultimate ontological status, things that are uncompounded - asaṃskṛta/asaṃkhata. One of the perished schools of Buddhism - the Sarvāstivāda was especially noted for the theory of uncompounded, sort of atomic entities called dharmas, time being also made out of minimal time-quantums that all exist simultaneously in the present (past and future as well).

Nāgārjuna is supposed to be the first, definitely the greatest early Mahayana-teacher and his teaching polemicizes precisely against this theory of uncompounded smallest entities, claiming, that they do not exist, that really everything, including Nirvāṇa, Pratītyasamutpāda and time is compounded. His teaching can also be read (thereby being truly mahayanic) as a rejection of self-centered hunger for enlightenment and thirst for Nirvāṇa.

Therefore, at least according to Mahayana the answer to your questions are yes, time IS compounded, we SHOULD disconnect ourselves from it, from clinging to it and we SHOULD freely spend it on others.

EDIT: replaced Hinayana by Theravada.