Gregorian Calendar to Thasathi Conversion

Following algorithm is used to convert Gregorian date into Thasathi Date.

Let Y=Year (CE), M=Month D=Date

Step 1: Convert Common Era into Thiruvalluvar Era. 

Thiruvalluvar Year Ty = Y+30 (M=1, D<15)

=Y+31 (Others)

Step 2: Determine Leap Index, 

L=0 for Normal Years

L=1 for Leap Years (Divisble by4, Not divisible by 100, Divisible by 400)

Step 3: For January Month,

Tm=13, D=D+14   (Before Jan 15)

Tm=1, D=D-14 (On or After Jan 15)

Step 4: For February Month,

Tm=1, D=D+17 (Before Feb 13+L)

Tm=2, D=D-12-L (On or After 13+L)

Gregorian Date Serial Number:

DSN= 30x(M-1) +D+E -->(1)

E is the additional day above 30 day months.

E=0 (Jan, Apr, May)

E=1 (Feb, June, July)

E=-1 (Mar)

E=2, Aug

E=3 (Sep, Oct)

E=4 (Nov,Dec)

Thasathi Date Serial Number:

TSN= DSN-14;

TSN=28*(Tm-1)+Td+1     -->(2)

Equating (1) and (2),

28*(Tm-1)+Td+1 +14= 30*(M-1)+D+E

28Tm-28+Td+15=30M-30+D+E

28Tm+Td=30M+D+E-17

28Tm+Td=30M+D+Z

Step 5: Determine Thasathi Date Index,

T=30*M+D-Z

where, Z=18 (Mar)

=17 (Apr, May)

=16 (June July)

=15(Aug)

=14 (Sep Oct)

=13 (Nov,Dec)

Step 6: Determine Thasathi Month and Date

Tm= Int((T)/28))

Td = 1+Rem ((T-1)/28)