A Request

[overweightninja]

Thanks a lot overweightninja!

Does anyone have any idea how I can implement that date as a trigger?

You need to use the condition "and I_TurnNumber > #". Remember to consider the four turns per year, so for example to add a condition of minimum year 260bc, we would do 272-260=12, 12*4 = 48.
So the end result would be:

Spoiler Alert, click show to read: 

;reset Celt counters
monitor_event FactionTurnEnd FactionIsLocal

if I_CompareCounter Celt_ForumCount > 5
set_counter Celt_Reform 1
end_if

if I_CompareCounter Celt_ForumCount > 9
and I_CompareCounter Celt_CityCount > 1
and I_CompareCounter Celt_TempleCount > 5
and I_TurnNumber > 48

set_counter Celt_Reform 2
end_if

if I_CompareCounter Celt_Reform < 2
set_counter Celt_ForumCount 0
set_counter Celt_CityCount 0
set_counter Celt_TempleCount 0
end_if
end_monitor

monitor_event FactionTurnStart FactionIsLocal
and I_CompareCounter Celt_ForumCount > 5
set_counter Celt_Reform 1
end_monitor

monitor_event FactionTurnStart FactionIsLocal
and I_CompareCounter Celt_ForumCount > 9
and I_CompareCounter Celt_CityCount > 1
and I_CompareCounter Celt_TempleCount > 5
and I_TurnNumber > 48
set_counter Celt_Reform 2
end_monitor