Sun rise/set without calculator :
For today.
1st Approximation
Sunrise is exactly at 6:00 in the morning.
Sunset at 6:00 in the evening.
But that would have been the case, if Dhaka lied exactly on the time zone line. It doesn't. Rather longitude is 90.4.
We know for every degree the sun travels, it takes 4 mins on average. Therefore for the extra 0.4 longitude, rise and set will occur 4*0.4 = 1.5 mins earlier. Round it to minus 2 mins.
-2 mins with sun rise and set. -- (correction 1)
Next, correction for season. Figure out Sun's altitude. Today is Christmas day, Winter solstice. Sun and Southern peak. Therefore altitude is -23.5 degree.
Now for every degree we need to multiply by 1.8 for Dhaka. Why? Shall try to explain later. Therefore -23*1.8=-42 minutes.
Therefore
+42 mins for rise.
-42 mins for set. -- (correction 2)
Lastly, sun "rise" or "set" isn't counted when the center of the sun is on horizon, rather the sun has to set, the upper arc has to touch horizon. Add 3 mins for that which accounts for refraction too.
+3 mins for sunset.
-3 mins for sunrise. -- (correction 3)
And now add equation of time with that. We know from the poem I shared previously
"On September one, trust the Sun.
Come Halloween, subtract sixteen.
On Christmas Day, you're OK.
For your Valentine true, add a dozen and two."
Today is Christmas day. "You are OK". Meaning equation of time 0.
+0 mins with both set and rise -- (correction 4)
Adding the above 4 corrections we find.
Sun rise 6:00 am - 2 mins + 42 mins - 3 mins + 0 mins = 6:37 am.
Sun set 6:00 pm - 2 mins - 42 mins + 3 mins + 0 mins = 5:19 pm.
Exact time we find on calendar for rise is 6:38 am and set at 5:19 pm. Our calculation is right.