The various past tenses in English permit temporal ordering and indications of having or not having finished. An adverbial indicating a temporal duration in or into the past acts as an additional marker to place the reader temporally (is there a way to talk about time non-spatially?). In academic writing this placement in time is usually specified by a date or period given in weeks, months, years, decades, centuries, millennia, … But sometimes a less specific time marker is appropriate, and it is these I consider here.
If the text has already established a period in the past, the adverbials then, back then, in those days, during those times, or at that time (alternatively, at this time if the reader can be considered immersed in the given time period rather than looking back in time) can be used to introduce a statement of how things were in the given period. These adverbials are used to signal a contrast in a past condition to a present one: At that time women did not have the vote. In those days there were no mobile phones. Back then seat belts were not mandatory. During those times, the economy was booming. At the time is like a past version of ‘at the moment’ or ‘currently’, not indicating contrast in the same way as ‘at that time’ but rather placing the reader at a particular point or in a past period of time: At the time it seemed like a good idea. At one time means ‘in the past’ without specifying the period in the past being referred to, i.e. ‘some time ago’ (unlike ‘at that time’ and ‘at the time’ where the time period if not spelled out is implicit): At one time starlings were commonly seen in this region. In the past focuses more on the contrast with the present than ‘at one time’: In the past I would walk to work, but now that I’ve moved to the outskirts I use public transport (cf. the idiom ‘all in the past’, meaning ‘no longer happening’, ‘no longer relevant’). Use of modifiers can help place the period in the past relative to the present: in the recent past, in the distant past, or in the not too distant past (the last conveying an element of it being more recent than one might think).
All these adverbials have an implied sense of completion – that was then, this is now. To indicate continuation up to the present, use ‘since’ or ‘from’ with an adverbial specifying a point P in the past (the former signals extension from P up to the present, the latter specifies P as a starting point for an ongoing action). For example from that time on, from then on, since that time, or since then. From that moment everyone realized what was happening. Since then the regulation of the market has been more stringent. From that time on she devoted her research to the fluctuation in bird migration patterns. Variants include from that day on, which pinpoints a significant event (on that day) causing a change in behaviour rather than the more gradual change in circumstance implied by ‘from that time on’ (if the ‘time’ in question is correspondingly vague – of course, if a specific point in time is being referred to, then the change is likewise more sudden: [He broke his leg.] From that time on he walked with a limp.). Ever since means continually or often from a past time: Ever since a supernova was first observed, researchers have been puzzling out an explanation for the phenomenon.
The adverbials long ago, a long time ago, a while ago, some time ago draw attention to the passing of time since the events in question (‘ago’ gives a sense of going back in time from the present) and for this reason are often used in story-telling where the auditor/reader is invited to cast their minds back in time (classically, once upon a time) and their imaginations into the frame of the story. But their use is not restricted to fictive contexts: Long ago dinosaurs roamed the Earth. I told you that a while ago. The more specific variants a few days ago, some weeks ago etc. are commonly used in factual texts. On the other hand, such phrases as back in the day, in time gone by, in days of yore hark back nostalgically to an often imaginary past.
