You are probably very familiar with the noun die Zeit, usually translated as "time," but may not know that there is also a preposition with the same spelling: zeit. We covered this to some extent in a previous lesson, but it's time for an in-depth recap. There is some potential confusion with expressions that use the noun die Zeit with a genitive modifier that we should look at first: 

Das wird die Zeit meines Lebens.

It will be the time of my life.

Caption 18, Glasperlenspiel - Geiles Leben

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This very straightforward expression, "having the time of your life," meaning that you are enjoying yourself very much, is common in English too. 

However, if you were to use the genitive preposition zeit in an expression that appears to be very similar, it would in fact mean something entirely different: 

Und trotzdem hatten Morisot und ihre drei Mitstreiterinnen zeit ihres Lebens mit ihrer Malerei Erfolg.

And Morisot and her three fellow campaigners were nevertheless successful all their lives with their painting.

Captions 73-74, Malerei - Impressionistinnen

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As you can see, the preposition zeit means, in a general way, "during the whole course of," so the literal translation "during the whole course of their lives" is more simply translated as "all their lives." The genitive form thus renders this standard expression in the following forms: 

— zeit meines Lebens ("all of my life")

— zeit deines Lebens ("all of your life")

— zeit Ihres Lebens ("all of your life")

— zeit seines Lebens ("all of his life," or for neuter subjects like animals, "all of its life")

— zeit ihres Lebens ("all of her life" or "all of their lives")

— zeit unseres Lebens ("all of our lives")

— zeit eures Lebens ("all of your lives")

Luckily, the preposition zeit is not commonly spoken, and nearly always in the above context when used at all. Remember that if the definite article die is used, then it's referring to having a good time, whereas the lack of definite article will be a good indication of its use referring to the span of a lifetime.

There's another expression for "all of your life" that may also appear a bit odd to the non-native German speaker: 

Da sammelte es sich die Taler hinein und war reich für seinen Lebtag.

Then she gathered up the thalers and was rich for the rest of her life.

Caption 35, Märchenstunde - Die Sterntaler

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You can remember the meaning of Lebtag more easily if you think of it as "the rest of the days of your life." If you are curious as to why es is translated here as "she" and seinen as "her," you should read the Yabla German lesson "Possessive Gender Benders." 

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Further Learning
Go to Yabla German to find other examples of how the noun die Zeit and the preposition zeit are used in real-world contexts, and read the full definition of the preposition zeit on Duden. 

We should all know the meaning of the noun die Zeit by now, considering it is also the title of one of the most well-known newspapers in Germany. But how many of you are familiar with the preposition zeit, written in lower-case? German, more so than English, tends to have many words and expressions that appear in print but are rarely spoken, and zeit is one of these. Let's start with the one we already know: 

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Ich wünschte, die Zeit würde stillstehen.

I wish that time could stand still.

Caption 40, Die Klasse - Berlin '61 - Part 8

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Für die Verschwörer wird die Zeit immer knapper.

For the conspirators, time gets scarcer and scarcer.

Caption 30, Die Stunde der Offiziere - Dokudrama über den 20. Juli 1944

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Das wird die Zeit meines Lebens

It will be the time of my life

Caption 18, Glasperlenspiel - Geiles Leben

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Note that in most cases, the German uses the definite article die (the) when referring to time, whereas English, in most cases, drops the article altogether. The last example above is interesting in that "the time of my life" is a standard expression, as is Die glücklichste Zeit meines Lebens ("The happiest time of my life"). This construction, as you will note, is very similar to the most standard use of the preposition zeit and it could be easy to confuse the two:

Ich bin Architekt von Beruf und habe zeit meines Lebens als Architekt gearbeitet.
I am an architect by profession and have worked as an architect for all of my life.

Zeit seines Lebens war er nie in Rom
During his lifetime he has never been to Rome.

In a general sense, zeit can be translated as "during" or "for." The first example of zeit meines Lebens could have been translated as "during my lifetime" or simply "all my life." Note that zeit meines Lebens therefore has a very different meaning from Die Zeit meines Lebens. The former is referring to something you have done all of your life, enjoyable or not, and the latter to having one of the best times of your life.

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Further Learning
In the unlikely event that you were to hear zeit spoken, you might easily think that it was the similar-sounding preposition seit. Remember, however, that seit is pronounced with an English sounding Z and takes the dative form, whereas zeit is pronounced with a Ts sound and takes the genitive form. See the full meaning of zeit on the German dictionary website Duden and see if you can find any other German expressions that use the preposition zeit.