To make a long answer short, in ancient and medieval eras it was believed that the Roman Emperor was the rightful direct ruler or overlord of all the world. People didn't believe that the Roman Empire was an empire, they believed it was THE EMPIRE and thus that the emperor was not an emperor but THE EMPEROR, the holder of a unique political position.
Thus whenever there were two or more persons claiming the position of emperor it was an ideological problem: https://en.wikipedia.org/wiki/Problem_of_two_emperors
So however much different Christians disagreed about who was the Roman Emperor, they agreed that the Roman Emperor was a unique ruler and there weren't any other emperors in Christiandom. Nobody ever talked or wrote aobut a Hungarian emperor or a Danish Empire or a Polish emperor.
It wasn't until 1721 that Tsar Peter the Great was proclaimed Russian Emperor, the first of a number of non Roman emperors and empires in European culture which I refer to as "inferators" instead of imperators, and as "inferiums" instead of imperiums.
So nobody ever wrote that the lands of the Bohemian Crown or the Lands of the Czech Crown ever constituated any sort of Bohemian or Czech or Praguian empire. There was never any state which officially claimed to be a Bohemian or Czech empire.
The Holy Roman Empire lasted for 844 or 1,006 years from 800 or 962 until 1806, and during that time, the courts and the administrative headquarters of the emperors often moved from one city or castle to another one. For a number of centuries the man who was elected Emepror of the Romans ws also the hereditary King of Bohemia, and some of them had their capitals at Prague in Bohemia, from 1347-1400, and from 1583 to 1612. Which is 53 plus 29 years, or 82 years in total.
Prague was the administrative center of an empire for only 82 years, and that empire was never called the Bohemian Empire,or the Czech Empire, or Praguian Empire. It was always called the Holy Empire, the Roman Empire, or the Holy Roman Empire, and its its ruler's imperial title was always plain "Emperor" or "Emperor of the Romans".
So the Bartimaeus series must be set in an alternate universe with a different history than our history if a Czech Empire ever existed in its history.
And the only information about when that alternate history diverged from ours would be various clues in the Bartimaeus series itself.
Long Answer with more details.
Trish's nswer is not complete nor perfectly accurate.
There never was a Bohemian or Czech Empire, only a Roman Empire in various forms.
The city state of Rome grew to rule the Mediterranean region, and the Roman Empire is considered to have begun in 27 BC.
The Romans claimed that their gods gave them the authority to rule the world, and the early Christians accepted that idea, only relacing the Roman gods with the Christian God.
In 395 the 2 sons of Theodosius I became rulers of the western and eastern sections of the Roman Empire. Barbarian groups took contol of the western section by 476 or 480, and those people in the west who were still loyal to the empire considered the eastern Emperor their overlord.
In the 530s Justinian in the east reconquered about a half of the western section of the Roman Empire. After more history, the empire was somewhat smaller in 797 when Irene, mother of Emperor Constantine VI, deposed and blinded him and ruled the Roman Empire herself.
Conservative people believed that a woman should't rule the empire and that the imperial throne was vacent. So Charlemagne, the mighty king of the Franks and the Lombards, had himself crowned emperor in Rome in 800, claiming to be the successor of Constantine VI and all the emperors back to Augustus. Irene was deposed by Nikephoros I in 802, and the eastern section of the Roman Empire continued until the final Turkish conquests in 1453, 1460, 1461, and 1475.
The Carolingian Empire fell apart, but in 962 Otto I the Great, the mighty KIng of Germany and Italy, was crowned Emperor in Rome by the pope. From then on the positions of king of Germany and Kng of Italy were united with the position of Emperor. In 1032 the position of King of Arles or Burgundy (western switzerland and southeastern France), became united with the position of Emperor.
The Emperors also ruled lands which were not in their three kingdoms, being the overlords of other lands, like the Czech lands, and sometimes the overlords of the kingdoms of Hungary, Poland, Denmark, etc. They claimed to be the rightful overlords of all the world, and that theoretical overlordship was sometimes acknowledged all over the western, central & northern, Latin, and Catholic, part of Europe.
The emperors owned many country villas and palaces and moved their courts and adminstrations with them as they moved from one residence to another.
During the 12th century the practice began that a man elected emperor used the title of Rex Romanorum et semper Augustus "King of the Romans and always Emperor" until and unless he was crowned emperor in Rome by the pope, when he would take the title of Imperator Romanorum et semper Augustus "Emperor of the Romans and always Emperor". The strong resemblence between the two titles was no doubt to show that the King of the Romans already had full imperial rights and powers.
In the mid 13th century, the practice of electing kings of the Romans - who rarely got to be crowned emperor - of diferent families than the previous ones, began and last for about 200 years. During that period the various kings and emperors of the Romans ruled from the capitals of their various fiefs and principalities.
(Some of the imperial institutions eventually had permanent homes. The reichskammergerich, founded in 1495, one of the two main imperial courts, moved between cities until settling in Speyer from 1527-1689, and in Wetzler from 1689 to 1806. The other main court of the Holy Roman Empire, the Aulic Council, founded in 1497, was at Vienna for most of the period 1497-1806. The imperial diet of the Empire sat at Regensburg continously from 1665 to 1803.)
In 1346 a group of Electors opposed to Emperor Louis IV, Duke of Bavaria, chose Charles, son of King John of Bohemia, as rival King of the Romans. Charles became King of Bohemia when his father was killed at the Battle of Crechy in 1346. After Louis died in 1347, Charles was relected king of the Romans, and crowned Emperor in 1355. His son Wencelaus IV was elected king of the Romans in 1376 and succeeded when Charles died in 1378.
Unfortunately for Wenceslaus, the Electors eventually decided that he was too much like "Bad KIng Wenceslaus" and in 1400 they declared Wencesalus deposed as King of the Romans, and elected Rupert, Count Palatine of the Rhine. Wenceslaus continued to claim to be King of the Romans until he died in 1419, but the Holy Roman Empire was ruled from Prague in Bohemia only in the 53 year period from 1347 to 1400.
Wenceslaus's half Brother Sigismund, King of Hungary, was elected King of the Romans in 1410 and 1411, and crowned Emperor of the Romans in 1433, no doubt because Hungary was between the Turks and central Europe. Sigismund was the rightful heir to Bohemia, but the followers of Jan Hus, burned at the stake despite a safe conduct from Sigismund, refused to accept Sigismund as king, causing the bloody Hussite wars. So Sigismund ruled the Holy Roman Empire from Budapest in Hungary instead of from Prague in Bohemia.
Sigismund died in 1437, leaving a daughter, Elizabeth, who was married to Duke Albert V of Austria. Albert became KIng of Hungary and Croatia in 1437, was elected KIng of the Romans 18 March 1438, and became KIng of Bohemia 6 May 1438, but never went to Bohemia, ruling from Hungary, and died 27 October 1439.
Albert's posthumous son Ladislaus Posthumous (1440-57) became Duke of Austria in 1440, King of Bohemia in 1453, and King of Hungary in 1440. Frederick, Duke of Styria, Carinthia, and Carniola, was elected King of the Romans because his lands were closest to the Turkish threat, became emperor in 1453, and inherited Austria in 1457.
In 1495 the diet of the Empire decreed that the Empire would now be called the Holy Roman Empire of the German Nation, but that addition was little used. Maximilian, KIng of the Romans, took the title of Elected Emperor of the Romans in 1509, and added King of Germany, perhaps to show that the kingdom was German and the Empire Roman.
In 1526 the Turks defeated and killed King Louis II of Hungary, Croata, and Bohemia at the Battled of Mohacs. Archduke Ferdinand of Austra became King of Bohemia and those parts of Hungary and Croatia the Turks didn't conquer.
In 1583 Rudolf II, Elected Emperor of the Romans, KIng of HUngary & Croatia, King of Bohemia, etc. moved his court to Prague in Bohemia. After Rudulf died in 1612 the court was gradually moved back to Vienna. So Prague was the imperial capital for another 29 years.
So Prague was the capital - or at least residence city of the emperor - of the Holy Roman Empire for a total of about 82 years during the 265 year period of 1347 to 1612, which contrasts with the 844, or 1,006, years that the Holy Roman Empire lasted.
There never was a state that officially called itself a Bohemian, Czech, or Praguian Empire.
The state named the Holy Roman Empire lasting for 1,006 or 844 years from 800 or 962 to 1806. For many centuries on and off the monarchs of the Holy Roman Empire using the title of King of the Romans, Emperor of the Romans, or Elected Emperor of the Romans, were also the Kings of Bohemia, with the capital of Bohemia at Prague. But only a few of the Emperors who were also kings of Bohemia made Prague their capital city for the Empire, for a total of only 82 years.
Fronn 1804 to 1918, The Kingdom of bohemia was part of the Empire of Austria, and the Emperor of Austria was also the King of Bohemia. But the capital of the Empire of Austria was always in Vienna, Austria, and never in Prague.
(And never write about the Austro-Hungarian Empire, which never existed. After the compromise of 1867, the Empire of Austria and the Kingdom of Hungrary were almost independent of each other, having a few institutions in common. The joint state was called Austrai-Hungary. But nobody ever used the title of Austro-Hungarian Emperor or called Austria-Hungary the Austro-Hungarian Empire).
So the Bartimaeus series must be set in an alternate universe with a different history than our history if a Czech Empire ever existed in its history.
And the only information about when that alternate history diverged from ours would be various clues in the Bartimaeus series itself.