Time adverbials play a part in specifying the time reference of the verb. For instance, in the following two sentences, it is the time adverbials ‘xiànzài’ (now) and ‘gāngcái’ (just now) that determine the time reference in (1a) is present, and in (1b) is past:
Háizimen xiànzài zài huāyuánlǐ wánr.
(The children are playing in the garden now.)
Dìdi gāngcái zài kètīnglǐ kàn diànshì.
(The younger brother was watching TV in the living room just now.)
In Mandarin Chinese adverbials of time have a wide range of grammatical realizations, especially notable are
1) noun phrases (including position words denoting time), like 过去‘guòqù’ (in the past), 以前/从前‘yǐqián/cóngqián’ (before), 上个月‘shàngge yuè’ (last month), 在上个圣诞节的时候‘zài shàng ge Shèngdànjié de shíhou’ (during last Christmas);
2) prepositional phrases, like ‘cóng qùnián Sānyuè dào Wǔyuè’ (from last March to May);
3) adverbs, like 曾/曾经‘céng/céngjīng’ (once), 已/已经‘yǐ/yǐjing’ (already), 将‘jiāng’(will), etc. Semantically, time expressions fall into three classes: those of time position, of duration (including span), and of time frequency. Some items belong to more than one class. Time expressions may take two positions with respect to the verbs they are related to, they can either precede the verb or follow the verb in accordance with the semantic roles they play. Their positions are governed by the PTS and Principle of Temporal Scope (PTSC). The PTSC proposed by Tai (1981: 214) states:
“If the conceptual state represented by a syntactic unit X falls within the temporal scope of the conceptual state represented by a syntactic unit Y, then the word order is YX.”
The PTSC, in other words, states that if the time span, within which an event takes place or a state obtains, falls within the time span represented by a time position adverbial, then the time position adverbial is ordered before the VP. PTSC is consistent with the Principle of Whole-Before-Part in that a time expression with a narrow temporal range follows the one with a broad temporal range.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.